Light-emitting semiconductor device producing red wavelength optical radiation

ABSTRACT

A light-emitting semiconductor device for producing red color optical radiation has a cladding layer of AlGaInPAs having a lattice constant between GaAs and GaP. Further, the laser diode uses an optical waveguide layer in the system of GaInPAs free from Al. The semiconductor device may be constructed on a GaPAs substrate.

This is a divisional of application Ser. No. 09/502,802 filed Feb. 11,2000, now U.S. Pat. No. 6,542,528.

BACKGROUND OF THE INVENTION

The present invention generally relates to light-emitting semiconductordevices and more particularly to a light-emitting semiconductor deviceincluding a laser diode that produces visible optical radiation in thewavelength band of red color.

The system of AlGaInP is a III-V material having a direct-transitiontype band structure and provides the bandgap of as much as about 2.3 eV(540 nm in terms of optical wavelength), which is the largest bandgapvalue of the III-V material system except for the system of AlGaInN orthe III-V material that contains B as the group III element. Thus,AlGaInP has been a target of intensive investigation in relation tohigh-power light emitting diode that produces visible optical radiationof green or red color. Such a high-power light emitting diodes of greento red color wavelength band has its application in color displaydevices. Further, the system of AlGaInP has been studied in relation tovisible-wavelength laser diode for use in laser printers or in opticalrecording of information, such as compact disk players or DVD players.

In the laser diode designed for producing red color wavelengthradiation, it has been practiced to use a material system that achievesa lattice matching with respect to a GaAs substrate. In the art ofhigh-density recording of information, in particular, there is a demandof a high-power laser diode that operates stably even in ahigh-temperature or unregulated temperature environment.

In a laser diode, laser oscillation is caused as a result of stimulatedemission taking place in an active layer of the laser diode, and as aresult of stimulated emission, an optical beam is produced in the activelayer. In order to achieve such a laser oscillation efficiently, it isnecessary to confine the carriers and further the optical radiation thusproduced in the active layer effectively, and for this purpose, acladding layer having a larger bandgap energy than the active layer isprovided in such a manner that the cladding layer is disposed adjacentto the active layer.

In an ordinary laser diode having a double-heterostructure, it has beenpracticed to use AlGaInP containing Al for the active layer in order toreduce the wavelength of the produced optical beam to the visiblewavelength band. It should be noted that Al thus added has an effect ofincreasing the bandgap energy of the active layer.

On the other hand, Al is a very reactive element and easily forms a deepimpurity level in the active layer by reacting with oxygen, which mayexist in the atmosphere used for growing the III-V epitaxial layer(s)constituting the laser diode. Further, such oxygen impurity may also becontained in the source material of the III-V crystal, although with atrace amount.

In view of the problems noted above, it is preferable to reduce the Alcontent in the epitaxial layer constituting the active layer of thelaser diode as much as possible. Thus, there is a proposal to use aGaInP quantum well for the active layer and sandwich the foregoing GaInPactive layer vertically by a pair of optical waveguide layers ofAlGaInP. Such a laser structure is called SCH-QW (separate confinementheterostructure quantum well) structure. Further, in relation to theSCH-QW laser diode, there is a proposal, as in the Japanese Laid-OpenPatent Publication 6-77592, to apply a strain to such a quantum welllayer constituting the active layer of the laser diode so as to decreasethe threshold of laser oscillation further. In the case of such a laserdiode using a strained quantum well structure for the active layer, thethickness of the quantum well layer is set to be smaller than a criticalthickness above which lattice relaxation takes place in the active layerby creating dislocations therein. It should be noted that such astrained quantum well layer is formed by choosing a material having alattice constant different from that of the substrate, for the quantumwell layer. In the case of the visible-wavelength laser diode thatoscillates at the visible wavelength such as the wavelength of 635 nm,it is indicated, in the Japanese Laid-Open Patent Publication 6-275915,that a tensile strain is more effective than a compressive strain. Inthe case of a quantum well layer formed on a GaAs substrate undertensile strain, the quantum well layer can take a composition of GaInP,which is closer to the GaP composition as compared with the substratecomposition of GaAs. Thereby, it should be noted that the quantum welllayer has an increased bandgap energy, and a quantum well layer having asuitable thickness can be used for the strained quantum well layer. Insuch a construction in which a sufficient thickness is secured for thequantum well layer, the adversary effect of interface defects issuccessfully avoided. On the other hand, the laser diode that uses thequantum well layer under tensile strain for the active layer operates inthe TM-mode, and the optical beam produced by such a laser diode has aplane of polarization which is 90° rotated as compared with the case ofusual laser diode operating in the TE-mode.

As noted before, the SCH-QW construction, which uses an opticalwaveguide layer typically having a composition of(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P, successfully achieves a desired opticalconfinement in the optical waveguide layer. On the other hand, such anoptical waveguide layer, containing a large amount of Al (x˜0.5)therein, has a drawback in that a damaging is tend to be caused at thefree edge surface of the laser cavity as a result of recombination ofthe carriers associated with the Al-induced defects contained in theoptical waveguide layer. Thus, such a laser diode has a drawback in thathigh-power operation is difficult. Further, such a laser diode has adrawback in that the reliability is degraded substantially when operatedfor a long period of time.

Further, such a SCH-QW has a drawback, particularly in the case itcontains a heterostructure of the AlGaInP system, in that the bandoffset is small in the side of the conduction band. More specifically,such a structure is characterized by a small band discontinuity (ΔE_(c))in the conduction band between the active layer and the cladding layer,and the carriers (electrons in particular) injected into the activelayer from the cladding layer easily cause an overflow. Thereby, thelaser diode shows a heavy temperature dependence in the laser thresholdcharacteristic, particularly the threshold current, and expensivetemperature regulation has been needed for stable operation thereof.This problem of poor temperature characteristic of the laser oscillationbecomes rapidly worse with decreasing oscillation wavelength. Forexample, the temperature dependence of the laser oscillationcharacteristic for the laser diode operating at the wavelength of 635 nmis much worse than that of the laser diode operating at the wavelengthof 650 nm.

In order to overcome the foregoing problem of temperature dependence ofthe laser diode, there is a proposal, as in the Japanese Laid-OpenPatent Publication 4-114486, to enhance the carrier confinementefficiency by providing a multiple quantum barrier (MQB) structurebetween the active layer and the cladding layer, wherein the MQBstructure includes a stacking of a number of extremely thin layers. Thisproposal, however, is turned out to be not realistic because of itsstructural complexity and the difficulty of thickness control of eachthin layer of the MQB structure. In order to obtain the desired effectaccording to this conventional approach, it is necessary to form theindividual layers of the MQB structure to be flat with the precision ofatomic layers.

Thus, there has been a distinct limitation in the improvement oftemperature dependence and further in the decrease of laser oscillationwavelength, as long as the laser diode is constructed on a conventionalGaAs substrate. More specifically, it has been not possible to realize alaser diode, according to such a conventional approach, that operates atthe wavelength of 635 nm in the operational environment of 80° C., withthe output optical power of 30 mW or more over a long duration such asten thousand hours or more.

In view of the foregoing problems associated with the use of the GaAssubstrate, there is a proposal to construct a laser diode on a GaPsubstrate. A GaP substrate has a smaller lattice constant as comparedwith a GaAs substrate and is thought more appropriate for the substratefor growing thereon epitaxial layers of the AlGaInP system. Thus, thereis a proposal in the Japanese Laid-Open Patent Publication 6-53602 of alaser diode constructed on a GaP substrate, wherein the laser diodeincludes a cladding layer of AlGaP having a composition ofAl_(y)Ga_(1-y)P (0≦y≦1) and a compressed active layer of GaInP having acomposition of Ga_(x)In_(1-x)P (0<x<1), which is a material having adirect-transition type band structure. In the proposed device, theactive layer is doped with N forming an isoelectronic trap. This priorart device, while being able to decrease the oscillation wavelength ofthe laser diode and simultaneously reduce the amount of Al contained inthe active layer, still has a drawback in that there remains a latticemisfit of as much as 2.3% with respect to the GaP substrate, even in thecase the active layer has a composition of Ga_(0.7)In_(0.3)P. It shouldbe noted that the GaInP mixed crystal maintains the direct-transitiontype band structure and has a lattice constant closest to the latticeconstant of the GaP substrate at the foregoing composition ofGa_(0.7)In_(0.3)P. The existence of the foregoing lattice misfit is notpreferable as such a lattice misfit reduces the critical thickness ofthe active layer. In order to avoid the creation of the lattice misfitdislocations in such a system, it is necessary to reduce the thicknessof the active layer significantly, while the use of such an extremelysmall thickness for the active layer is not practical.

Further, there is a proposal in the Japanese Laid-Open PatentPublication 5-41560 in which a double heterostructure including anAlGaInP active layer and an AlGaInP cladding layer is formed on a GaAssubstrate with an intervening buffer layer of GaPAs, wherein the AlGaInPlayers constituting the double heterostructure have a composition of(AlGa)_(a)In_(1-a)P (0.51<a≦0.73) and a corresponding lattice constantintermediate to the lattice constant of GaAs and the lattice constant ofGaP. The buffer layer has a composition represented as GaP_(x)As_(1-x)and eliminates the lattice misfit between the GaAs substrate and thedouble heterostructure.

FIG. 1 shows the relationship between the bandgap energy and compositionin the AlGaInP system used in the foregoing prior art, wherein thecontinuous lines of FIG. 1 represent the composition that provides thedirect-transition type band structure, while the broken lines representthe composition that provides the indirect-transition type bandstructure.

Referring to FIG. 1, the foregoing composition of AlGaInP((AlGa)_(a)In_(1-a)P; 0.51<a≦0.73) having the intermediate latticeconstant between GaAs and GaP falls in the region defined by thecomposition AlInP and the composition GaInP, wherein the AlInPcomposition is an intermediate composition on the line connecting theAlP composition and the InP composition. Further, the GaInP compositionis an intermediate composition on the line connecting the GaPcomposition and the InP composition.

According to the foregoing approach of the Japanese Laid-Open PatentPublication 5-41560, it is possible to use the AlGaInP compositioncharacterized by a lager bandgap as compared with the material systemthat achieves a lattice matching with the GaAs substrate for thecladding layer or active layer of the laser diode. Thus, the foregoingprior art is advantageous for realizing a laser diode operating at thevisible wavelength of 600 nm or shorter. This wavelength bandcorresponds to green to yellow color radiation.

On the other hand, the foregoing prior art structure is not suitable forthe laser diode operable at the wavelength of 635 nm or 650 nmcorresponding to red color radiation. For example, it can be seen fromFIG. 1 that the bandgap energy changes with lattice constant in thesystem of GaInP with a steeper ratio as compared with the system ofAlInP up to the composition in which the Ga content is 0.73, asrepresented in FIG. 1 by a continuous line. Thus, when the compositionof the GaInP active layer is tuned to the wavelength of 635 nm, thelattice constant of the active layer takes a value close to that of theGaAs substrate. On the other hand, it is preferable to increase thebandgap energy of the AlInP cladding layer as much as possible foreffective confinement of the carriers. In order to do this, it ispreferable to choose the composition close to AlP for the claddinglayer. However, such a selection of the cladding layer compositioninvites a heavy accumulation of compressive strain in the active layer.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful light-emitting semiconductor device wherein theforegoing problems are eliminated.

Another and more specific object of the present invention is to providea light-emitting laser diode including a laser diode operable in ahigh-temperature environment at the visible wavelength band of red suchas 635 nm or 650 nm.

Another object of the present invention is to provide an efficientlight-emitting semiconductor device, including laser diode andlight-emitting diode, operable in the room temperature environment atthe visible wavelength band of 600 nm or shorter.

Another object of the present invention is to provide a light-emittingsemiconductor device, comprising:

a semiconductor substrate;

an active layer provided above said semiconductor substrate, said activelayer producing a red optical radiation; and

a cladding layer provided above said semiconductor substrate adjacent tosaid active layer,

said active layer comprising a III-V material in the system of AlGaInPAshaving a composition represented as(Al_(x)Ga_(1-x))_(a)In_(1-a)P_(t)As_(1-t)(0x<1, 0<α1, 0 t 1),

said cladding layer containing Al and comprising a III-V material in thesystem of AlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(a)In_(1-β)P_(v)As_(1-v) (0<y1, 0.5<β1, 0<V 1), saidcladding layer having a bandgap larger than a bandgap of said activelayer and a lattice constant intermediate between a lattice constant ofGaP and a lattice constant of GaAs.

Another object of the present invention is to provide a light-emittingsemiconductor device, comprising:

a semiconductor substrate;

an active layer provided above said semiconductor substrate, said activelayer producing a red optical radiation;

a cladding layer provided above said semiconductor substrate adjacent tosaid active layer; and

an optical waveguide layer interposed between said active layer and saidcladding layer,

said active layer comprising a single quantum well layer of a III-Vmaterial in the system of AlGaInPAs having a composition represented as(Al_(x)Ga_(1-x))_(a)In_(1-a)P_(t)As_(1-t) (0≦x<1, 0<α≦1; 0≦t≦1),

said cladding layer containing Al and comprising a III-V material in thesystem of AlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β≦1, 0<v≦1), saidcladding layer having a bandgap larger than a bandgap of said activelayer and a lattice constant intermediate between a lattice constant ofGaP and a lattice constant of GaAs,

said optical waveguide layer comprising a III-V material in the systemof AlGaInPAs having a composition represented as(Al_(z)Ga_(1-z))_(a)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ≦1, 0<u≦1), saidoptical waveguide layer having a bandgap larger than said bandgap ofsaid active layer but smaller than said bandgap of said cladding layer.

Another object of the present invention is to provide a light-emittingsemiconductor device, comprising:

a semiconductor substrate;

an active layer provided above said semiconductor substrate, said activelayer producing a red optical radiation;

a cladding layer provided above said semiconductor substrate adjacent tosaid active layer; and

an optical waveguide layer interposed between said active layer and saidcladding layer,

said active layer having a multiple quantum well structure comprising aplurality of quantum well layers of a III-V material in the system ofAlGaInPAs having a composition represented as (Al_(x1)Ga_(1-x1))_(α1)In_(1-α1) P_(t1)As_(1-t1) (0≦x1<1, 0<α1≦1, 0≦t1≦1) and a plurality ofbarrier layers of a III-V material in the system of AlGaInPAs having acomposition represented as(Al_(x2)Ga_(1-x2))_(α2)In_(1-α2)P_(t2)As_(1-t2) (0≦x2<1, 0<α2<1,0≦t2≦1), each of said barrier layers having a bandgap larger than abandgap of said quantum well layer,

said cladding layer containing Al and comprising a III-V material in thesystem of AlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β≦1, 0<v≦1), saidcladding layer having a bandgap larger than a bandgap of said quantumwell layer in said active layer and a lattice constant intermediatebetween a lattice constant of GaP and a lattice constant of GaAs,

said optical waveguide layer comprising a III-V material in the systemof AlGaInPAs having a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ≦1, 0<u≦1), saidoptical waveguide layer having a bandgap larger than said bandgap ofsaid quantum well layer in said active layer but smaller than saidbandgap of said cladding layer.

According to the present invention, an efficient laser oscillation isobtained at the visible, red optical wavelength by using a claddinglayer having a lattice constant intermediate between GaAs and GaP. Itshould be noted that the system of AlGaInP having such a latticeconstant can provide a large bandgap effective for carrier confinementwhile reducing the Al-content therein. Further, the present inventionenables of using an Al-free composition for the optical waveguide layersin producing a red wavelength beam. Thereby, the laser diode can beoperated at a high output power without causing damage on the opticalcavity edge surface. By introducing As into the cladding layer, itbecomes possible to suppress the hillock formation.

Another object of the present invention is to provide a laser diode,comprising:

a semiconductor substrate;

a first cladding layer of AlGaInP provided above said semiconductorsubstrate, said first cladding layer having a first conductivity typeand a lattice constant intermediate between a lattice constant of GaAsand a lattice constant of GaP;

an active layer of GaInPAs provided above said first cladding layer;

a second cladding layer of AlGaInP provided above said active layer,said second cladding layer having a second conductivity type and alattice constant substantially identical with said lattice constant ofsaid first cladding layer;

an etching stopper layer of GaInP provided above said second claddinglayer, said etching stopper layer having said second conductivity type;

a third cladding layer of AlGaInP provided above said etching stopperlayer, said third cladding layer having said second conductivity typeand a lattice constant substantially identical with said latticeconstant of said first cladding layer;

said etching stopper layer having a lattice constant generally equal tosaid lattice constant of said first cladding layer and a bandgapsubstantially larger than a bandgap of said active layer.

Another object of the present invention is to provide a method offabricating a laser diode, comprising the steps of:

forming a layered structure, on a semiconductor substrate, such thatsaid layered structure includes a first cladding layer of AlGaInPprovided above said semiconductor substrate, said first cladding layerhaving a first conductivity type and a lattice constant intermediatebetween a lattice constant of GaAs and a lattice constant of GaP; anactive layer of GaInPAs provided above said first cladding layer; asecond cladding layer of AlGaInP provided above said active layer, saidsecond cladding layer having a second conductivity type and a latticeconstant substantially identical with said lattice constant of saidfirst cladding layer; an etching stopper layer of GaInP provided abovesaid second cladding layer, said etching stopper layer having saidsecond conductivity type; a third cladding layer of AlGaInP providedabove said etching stopper layer, said third cladding layer having saidsecond conductivity type and a lattice constant substantially identicalwith said lattice constant of said first cladding layer; said etchingstopper layer having a lattice constant generally equal to said latticeconstant of said first cladding layer and a bandgap substantially largerthan a bandgap of said active layer,

etching said third cladding layer to form a stripe region while usingsaid etching stopper layer as an etching stopper, until said etchingstopper layer is exposed at both lateral sides of said stripe region;and

filling a current confinement region on said exposed etching stopperlayer at both lateral sides of said stripe region.

Another object of the present invention is to provide a method offabricating a laser diode, comprising the steps of:

forming a layered structure, on a semiconductor substrate, such thatsaid layered structure includes a first cladding layer of AlGaInPprovided above said semiconductor substrate, said first cladding layerhaving a first conductivity type and a lattice constant intermediatebetween a lattice constant of GaAs and a lattice constant of GaP; anactive layer of GaInPAs provided above said first cladding layer; asecond cladding layer of AlGaInP provided above said active layer, saidsecond cladding layer having a second conductivity type and a latticeconstant substantially identical with said lattice constant of saidfirst cladding layer; an etching stopper layer of GaInP provided abovesaid second cladding layer, said etching stopper layer having saidsecond conductivity type and a lattice constant generally equal to saidlattice constant of said first cladding layer, said etching stopperlayer having a bandgap substantially larger than a bandgap of saidactive layer; and a current-confinement layer provided above saidetching stopper layer, said current-confinement layer having said firstconductivity type;

etching said current confinement layer to form a stripe opening whileusing said etching stopper layer as an etching stopper, until saidetching stopper layer is exposed along said stripe opening; and

depositing a third cladding layer of AlGaInP having said secondconductivity type on said current-confinement layer so as to fill saidstripe opening.

According to the present invention, it is possible to form acurrent-confinement structure by a wet etching process by using anetching stopper of GaInP while avoiding unwanted optical radiation inthe wavelength of red optical radiation by sun an etching stopper layer.

Another object of the present invention is to provide a light-emittingsemiconductor device, comprising:

a semiconductor substrate;

a first cladding layer of n-type AlGaInP provided above saidsemiconductor substrate, said first cladding layer having a compositionrepresented as (Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)P (0≦x1, 0.51<y1≦1) and alattice constant intermediate between a lattice constant of GaAs and alattice constant of GaP;

an active layer provided above said first cladding layer;

a second cladding layer of p-type AlGaInP provided above said activelayer, said second cladding layer having a composition substantiallyidentical with said composition of said first cladding layer;

wherein a multiple quantum barrier structure is interposed between saidactive layer and said second cladding layer,

said multiple quantum barrier structure comprising an alternaterepetition of a quantum well layer having a composition represented as(Al_(x2)Ga_(1-x2))_(y2)In_(1-y2)P (0≦x2≦1, 0≦y1≦1) and a bandgap smallerthan a bandgap of said second cladding layer, and a barrier layer havinga composition substantially identical with said composition of saidsecond cladding layer.

Another object of the present invention is to provide a light-emittingsemiconductor device, comprising:

a semiconductor substrate;

a first cladding layer of n-type AlGaInP provided above saidsemiconductor substrate, said first cladding layer having a compositionrepresented as (Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)P (0≦x1≦1, 0.51<y1≦1) anda lattice constant intermediate between a lattice constant of GaAs and alattice constant of GaP;

an active layer provided above said first cladding layer;

a second cladding layer of p-type AlGaInP provided above said activelayer, said second cladding layer having a composition substantiallyidentical with said composition of said first cladding layer;

wherein a carrier blocking layer is interposed at least between saidactive layer and said second cladding layer, said carrier blocking layerhaving a composition represented as (Al_(x3)Ga_(1-x3))_(y1)In_(1-y1)P(0≦x1≦x3≦1, 0.51<y1≦1) and a bandgap larger than a bandgap of saidsecond cladding layer, said carrier blocking layer having a latticeconstant generally matching with a lattice constant of said secondcladding layer.

Another object of the present invention light-emitting semiconductordevice, comprising:

a semiconductor substrate;

a first cladding layer of n-type AlGaInP provided above saidsemiconductor substrate, said first cladding layer having a compositionrepresented as (Al_(x1)Ga_(1-x1))_(y1)In_(1-y1)P (0≦x1≦1, 0.51<y1≦1) anda lattice constant intermediate between a lattice constant of GaAs and alattice constant of GaP;

an active layer provided above said first cladding layer;

a second cladding layer of p-type AlGaInP provided above said activelayer, said second cladding layer having a composition substantiallyidentical with said composition of said first cladding layer;

wherein a carrier blocking layer is interposed at least between saidactive layer and said second cladding layer, said carrier blocking layerhaving a composition represented as (Al_(x4)Ga_(1-x4))_(y4)In_(1-y4)P(0x4 1, 0.51<y1<y4 1) and a bandgap larger than a bandgap of said secondcladding layer, said carrier blocking layer having a lattice constantsmaller than a lattice constant of said second cladding layer.

According to the present invention, an efficient light-emittingsemiconductor device producing red color radiation is obtained by usinga carrier blocking layer that blocks the carriers injected into theactive layer from causing overflowing.

Other objects and further features of the present invention will becomeapparent from the following detailed description when red in conjunctionwith the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the band diagram of the AlGaInPAs system;

FIG. 2 is a diagram showing the principle of the present invention;

FIG. 3 is another diagram showing the principle of the presentinvention;

FIGS. 4A and 4B are further diagrams showing the principle of thepresent invention;

FIGS. 5A and 5B are further diagrams showing the principle of thepresent invention;

FIG. 6 is a further diagram showing the principle of the presentinvention;

FIG. 7 is a diagram showing the structure of a laser diode according tofirst through seventh embodiments of the present invention;

FIG. 8 is a diagram showing the laser diode of the fourth embodiment inan enlarged scale;

FIG. 9 is a diagram showing the laser diode of the sixth and seventhembodiments in an enlarged scale;

FIG. 10 is a diagram showing the construction of a laser diode accordingto an eighth embodiment of the present invention;

FIG. 11 is a diagram showing the construction of a laser diode accordingto a ninth embodiment of the present invention;

FIG. 12 is a diagram showing the construction of a laser diode accordingto a tenth embodiment of the present invention;

FIG. 13 is a diagram showing the construction of a laser diode accordingto an eleventh embodiment of the present invention;

FIG. 14 is a diagram showing the construction of a laser diode accordingto a twelfth embodiment of the present invention;

FIG. 15 is a diagram showing the construction of a laser diode accordingto a thirteenth embodiment of the present invention;

FIG. 16 is a diagram showing the construction of a laser diode accordingto a fourteenth embodiment of the present invention;

FIG. 17 is a band diagram explaining the principle of a fifteenthembodiment of the present invention;

FIG. 18 is a diagram showing the construction of a laser diode accordingto the fifteenth embodiment of the present invention;

FIG. 19 is a band diagram showing the band structure of the laser diodeof FIG. 18;

FIG. 20 is a diagram showing the construction of a laser diode accordingto a sixteenth embodiment of the present invention;

FIG. 21 is a band diagram showing the band structure of the laser diodeof FIG. 20;

FIG. 22 is a diagram showing the construction of a laser diode accordingto a seventeenth embodiment of the present invention;

FIG. 23 is a band diagram showing the band structure of the laser diodeof FIG. 22;

FIG. 24 is a band diagram showing the band structure of a laser diodeaccording to an eighteenth embodiment of the present invention; and

FIG. 25 is a band diagram showing the band structure of a laser diodeaccording to a nineteenth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[Principle]

First, the principle of the present invention will be explained withreference to FIGS. 2-4.

Referring to FIG. 2, the present invention provides, in a first aspectthereof, a light-emitting semiconductor device, comprising a GaAssubstrate 1, an active layer 3 provided above the semiconductorsubstrate 1 for producing a red optical radiation, and a pair ofcladding layers 3 provided above the semiconductor substrate 1 at thetop side and bottom side of the active layer 2, wherein the active layer2 is formed of a III-V material in the system of AlGaInPAs having acomposition represented as (Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t)(0≦x<1, 0<α≦1, 0t≦1), while the cladding layers 3 contain Al andcomprise a III-V material in the system of AlGaInPAs having acomposition represented as (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v)(0<y≦1, 0.5<β≦1, 0<v≦1). Thereby, there is formed a heterojunctionregion 4 in the laser diode from the active layer 2 and the top andbottom cladding layers 3. Each of the cladding layers 3 has a bandgaplarger than a bandgap of said active layer 2 and a lattice constantintermediate between a lattice constant of GaP and a lattice constant ofGaAs.

According to the present invention, the cladding layers 3 of AlGaInPAshaving the composition of (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v)(0<y≦1, 0.5<β≦1, 0<v≦1) contain Al therein and thus, the semiconductordevice, which may be a laser diode, can produce a short, visiblewavelength radiation with a high efficiency as compared with the case inwhich a III-V material achieving a lattice matching with respect to theGaAs substrate 1 is used for the cladding layer. Moreover, the use ofthe active layer having the composition of(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (0≦x<1, 0<α≦1, 0≦t≦1) in thepresent invention is advantageous for narrowing the bandgap as comparedwith the conventional laser diode designed for producing a green toyellow radiation at the wavelength shorter than 600 nm. Thereby, thesemiconductor device of the present invention can produce a red opticalradiation, longer in wavelength than 600 nm, efficiently. Further, theactive layer 2 having the foregoing composition can accumulate a straintherein. This feature is also advantageous for the semiconductor deviceto produce a red optical radiation.

In a second aspect, the present invention provides a light-emittingsemiconductor device similar to the device of the first mode except thata pair of optical waveguide layers 5 are interposed each between theactive layer 2 and the top or bottom cladding layer 4 and that theactive layer 2 is formed as a single quantum well layer of a III-Vmaterial in the system of AlGaInPAs having a composition represented as(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (0≦x<1, 0<α≦1, 0≦t≦1). Thecladding layers 3 contain Al and are formed of a III-V material in thesystem of AlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β≦1, 0<v≦1),similarly as before. The cladding layers 3 have a bandgap larger than abandgap of the active layer 2 and a lattice constant intermediatebetween a lattice constant of GaP and a lattice constant of GaAs. Theoptical waveguide layers 5 comprise a III-V material in the system ofAlGaInPAs having a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ≦1, 0<u≦1),wherein each of the optical waveguide layers 5 has a bandgap larger thansaid bandgap of said active layer but smaller than the bandgap of thecladding layer 3.

According to the present invention, the cladding layers 3 of AlGaInPAshaving the composition of (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v)(0<y≦1, 0.5<β≦1, 0<v≦1) contain Al therein and thus, the semiconductordevice, which may be a laser diode, can produce a short, visiblewavelength radiation with a high efficiency as compared with the case inwhich a III-V material achieving a lattice matching with respect to theGaAs substrate 1 is used for the cladding layer. Moreover, due to theSCH structure formed by the active layer 2 having the composition of(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (0≦x<1, 0<α≦1, 0≦t≦1) and thetop and bottom optical waveguide layers 5 having the composition of(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ≦1, 0<u≦1), awide bandgap can be realized with a lesser amount of Al for the claddinglayer 3 or the optical waveguide layer 5 as compared with thecomposition that achieves a lattice matching to the GaAs substrate 1.Thereby, the problem of deterioration at the edge of the cladding layer3 or the optical waveguide layer 5 is substantially reduced and thesemiconductor device can be used as a high-power laser diode. Further,it is possible to provide a strain to the active layer 2 by adjustingthe composition thereof with respect to the cladding layer 3. Thereby,it is possible to tune the bandgap to the desired red optical radiation.

In the system of GaInP, it is known that the band gap becomes largerwith increasing amount of Ga therein (Sandip, et al., Appl. Phys. Lett.60, 1992, pp. 630-632), while such a change of the bandgap induces achange of band discontinuity at the conduction band or at the valenceband. In the case of the GaInP system, the change of band discontinuitytakes place primarily on the conduction band and no substantial changeoccurs on the valence band. On the other hand, addition of Al to theforegoing GaInP system causes an increase of the conduction band energyand a decrease of the valence band energy, wherein the change of thevalence band energy is much larger than the change of the conductionband energy.

Conventionally, a laser diode constructed on a GaAs substrate has usedan optical waveguide layer of AlGaInP containing a large amount of Al.Associated with this, there has been a large band discontinuity in sucha conventional layer diode between the quantum well layer of GaInP andthe optical waveguide layer. On the other hand, such a conventionallaser diode has suffered from the problem of insufficient banddiscontinuity on the conduction band, and hence the problem of poortemperature characteristic. In the laser diode of the present invention,it is possible to reduce the Al content in the optical waveguide layerwhile maintaining a large bandgap, and a large band discontinuity can besecured for the conduction band at the interface between the activelayer 2 and the optical waveguide layer 5. Thereby, the problem ofcarrier (electron) overflow in the conventional red color laser diode issuccessfully eliminated. See FIGS. 4A and 4B comparing the bandstructure of the active layer according to the present invention (FIG.4B) in comparison with a related art (FIG. 4A) that uses GaInP for theactive layer 2 in combination with the optical waveguide layer 3 ofAlGaInP. In the example of FIGS. 4A and 4B, it should be noted that thepresent invention represented in FIG. 4B uses the composition of GaInPfor the optical waveguide layer 5, wherein the foregoing compositionfalls in the composition of (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u)(0≦z<1, 0.5<γ≦1, 0<u≦1).

Due to the increased degree of freedom of design, the laser diode of thepresent invention provides an improved performance not only in thewavelength band shorter than 600 nm but also in the wavelength bandlonger than 600 nm.

In a third aspect, the present invention provides a light-emittingsemiconductor device similar to the device of the first or second aspectof the present invention explained before, wherein the active layer 2now has a multiple quantum well structure comprising a plurality ofquantum well layers of a III-V material in the system of AlGaInPAshaving a composition represented as(Al_(x1)Ga_(1-x1))_(α1)In_(1-α1)P_(t1)As_(1-t1) (0≦x1<1, 0<α1≦1, 0 t1≦1)and a plurality of barrier layers of a III-V material in the system ofAlGaInPAs having a composition represented as(Al_(x2)Ga_(1-x2))_(α2)In_(1-α2)P_(t2)As_(1-t2) (0≦x2<1, 0<α2<1,0≦t1≦1), wherein each of the barrier layers has a bandgap larger than abandgap of said quantum well layer. The cladding layers 3 contain Al andare formed of a III-V material in the system of AlGaInPAs having acomposition represented as (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v)(0<y≦1, 0.5<β≦1, 0<v≦1), wherein the cladding layers 3 have a bandgaplarger than a bandgap of the quantum well layer in the active layer 2and a lattice constant intermediate between a lattice constant of GaPand a lattice constant of GaAs. The optical waveguide layer comprises aIII-V material in the system of AlGaInPAs having a compositionrepresented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1,0.5<γ≦1, 0<u≦1), wherein the optical waveguide layer 5 has a bandgaplarger than the bandgap of the quantum well layer in the active layer 2but smaller than the bandgap of said cladding layer 3.

According to the present invention, the cladding layers 3 of AlGaInPAshaving the composition of (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v)(0<y≦1, 0.5<β≦1, 0<v≦1) contain Al therein and thus, the semiconductordevice, which may be a laser diode, can produce a short, visiblewavelength radiation with a high efficiency as compared with the case inwhich a III-V material achieving a lattice matching with respect to theGaAs substrate 1 is used for the cladding layer. Moreover, due to theSCH structure formed by the active layer 2 having the composition of(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (0≦x<1, 0<α≦1, 0≦t≦1) and thetop and bottom optical waveguide layers 5 having the composition of(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ≦1, 0<u≦1), awide bandgap can be realized with a lesser amount of Al for the claddinglayer 3 or the optical waveguide layer 5 as compared with thecomposition that achieves a lattice matching to the GaAs substrate 1.Thereby, the problem of deterioration at the edge of the cladding layer3 or the optical waveguide layer 5 is substantially reduced and thesemiconductor device can be used as a high-power laser diode. Further,it is possible to provide a strain to the quantum well layersconstituting the active layer 2 by adjusting the composition thereofwith respect to the cladding layer 3. Thereby, it is possible to tunethe bandgap to the desired red optical radiation. For example, it ispossible to apply a compressive strain to the quantum well layers and atensile strain to the barrier layers, or vice versa.

As a result of the use of the multiple quantum well structure, it ispossible to confine the carriers effectively into the quantum welllayers according to the present invention. Further, a narrow bandgapmaterial can also be used for the quantum well layers in such a multiplequantum well active layer 2.

As noted already, it is known in the system of GaInP that the band gapbecomes larger with increasing amount of Ga therein (Sandip, et al., opcit.), while such a change of the bandgap induces a change of banddiscontinuity at the conduction band or at the valence band. In the caseof the GaInP system, the change of band discontinuity takes placeprimarily on the conduction band and no substantial change occurs on thevalence band. On the other hand, addition of Al to the foregoing GaInPsystem causes an increase of the conduction band energy and a decreaseof the valence band energy, wherein the change of the valence bandenergy is much larger than the change of the conduction band energy.

Conventionally, a laser diode constructed on a GaAs substrate has usedan optical waveguide layer of AlGaInP containing a large amount of Al.Associated with this, there has been a large band discontinuity in sucha conventional layer diode between the quantum well layer of GaInP andthe optical waveguide layer 5 of AlGaInP. On the other hand, such aconventional laser diode has suffered from the problem of insufficientband discontinuity in the conduction band, and hence the problem of poortemperature characteristic. In the laser diode of the present invention,it is possible to reduce the Al content in the optical waveguide layer 5while maintaining a large bandgap, and a large band discontinuity can besecured for the conduction band at the interface between the activelayer 2 and the optical waveguide layer 5. Thereby, the problem ofcarrier (electron) overflow in the conventional red color laser diode issuccessfully eliminated. See FIG. 4 comparing the band structure of theactive layer according to the present invention in comparison with arelated art that uses GaInP for the active layer 2 in combination withthe optical waveguide layer 3 of AlGaInP. In the example of FIG. 4, itshould be noted that the present invention represented at the right usesthe composition of GaInP for the optical waveguide layer 5, wherein theforegoing composition falls in the composition of(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ≦1, 0<u≦1).

Due to the increased degree of freedom of design, the laser diode of thepresent invention provides an improved performance not only in thewavelength band shorter than 600 nm but also in the wavelength bandlonger than 600 nm.

In any of the foregoing first through third aspects of the presentinvention, it is preferable that the active layer 2 contains As. Byincorporating As into the active layer 2, the bandgap of the activelayer 2 is tuned to the wavelength of red optical radiation such as 635nm or 650 nm. Thereby, the band discontinuity in the conduction band isincreased and the problem of carrier overflow is minimized. The laserdiode having such an active layer selected from the AlGaInPAs system andcontaining As operates efficiently even in a unregulated temperatureenvironment. The active layer containing As has a compositionrepresented as (Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (0≦x<1, 0<α≦1,0≦t<1).

Further, it is preferable to choose the composition of the opticalwaveguide layer 5 to be free from Al in any of the first through thirdaspects of the present invention. Even in such a case in which theoptical waveguide layer is free from Al, it is possible to secure alarge bandgap for the optical waveguide layer 5, provided that theoptical waveguide layer 5 has a lattice constant smaller than thelattice constant of GaAs. See the band diagram of FIG. 1. For example, abandgap wavelength of 570 nm, which is normally achieved by the Al-richcomposition of (Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P, is successfullyachieved in the present invention by the Al-free optical waveguide layer5 having a composition of Ga_(0.7)In_(0.3)P, wherein the GaInP havingsuch a composition has a lattice constant between the lattice constantof GaP and the lattice constant of GaAs. Thereby, the laser diodeoperable at the red optical wavelength of 635 nm or 650 nm can beconstructed by using Al-free III-V epitaxial layers. Due to theelimination of non-optical recombination of carriers associated with Al,the laser diode free from Al or containing smaller amount of Alaccording to the present invention can be used stably at a high opticaloutput power. The optical waveguide layer 5 has an Al-free compositionrepresented as Ga_(γ)In_(1-γ)P_(u)As_(1-u) (0.5<γ≦1, 0<u≦1).

Further, in any of the first through third aspects of the presentinvention, it is preferable that the cladding layer 3 contains As andhas a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β≦1, 0<v<1).

When growing a layer of AlGaInP on a (100) surface or a surface slightlyinclined from the (100) surface of a GaP, GaAs, or a GaP_(0.4)As_(0.6)substrate by means of an MOCVD process, it has been discovered thatthere occurs an extensive hillock formation on the surface of theAlGaInP crystal layer thus grown. This hillock formation is particularlysignificant in the case the AlGaInP layer that contains a large amountof Al, as in the case of growing an AlInP layer. Naturally, such ahillock formation decreases the yield and is not desirable in view pointof device fabrication process. It is believed that the droplets of Al orGa formed during the growth of the AlGaInP layer acts as the nuclei ofthe hillock structure.

In the experimental investigation constituting a foundation of thepresent invention, the inventor of the present invention has discoveredthat the hillock density is drastically reduced by merely incorporatinga small amount of As in to the AlGaInP layer.

FIG. 5A shows the surface morphology of an AlInP layer grown epitaxiallyon a GaPAs substrate while FIG. 5B shows the surface morphology of anAlInPAs layer grown epitaxially on the same GaPAs substrate, wherein theGaPAs substrate has a composition of GaP_(0.4)As_(0.6) and an inclinedcrystal surface offset from the (100) surface in the [110] direction byan angle of 2°, on which the growth of the AlGaInP layer or theAlGaInPAs layer was made. An MOCVD process was used and the substratetemperature was set to 750°.

Referring to FIG. 5A, it can be seen that there occurs a substantialhillock formation on the substrate, while in the case of FIG. 5B, thehillock formation is entirely eliminated by merely incorporating As intothe epitaxial layer. In the experiment of FIG. 5B, the epitaxial layercontained only 10% of As.

Thus, as set forth above, it is desirable to incorporate As into thecladding layer 3 in any of the first through third aspects of thepresent invention for hillock-free growth of the same. As noted before,the composition of the cladding layer 3 is represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β≦1, 0<v<1).

Further, in any of the foregoing first through third aspects of thepresent invention, it is preferable to set the composition of thequantum well layer or the active layer 2 to have a lattice constantlarger than a lattice constant of the cladding layer 3.

In the laser diode of related art constructed on a GaAs substrate foroperation at the wavelength of 635 nm, a tensile strain has been usedfor the quantum well layer 2 in view of the very small bandgap energy ofthe material constituting the quantum well layer 2. In order toaccumulate a compressive strain in such a system, it should be notedthat a narrow gap material such as GaInP has to be used. In order tosecure a sufficiently large transition energy tuned to the opticalwavelength of 635 nm in such a laser diode that uses a narrow gapmaterial for the active layer, it has been necessary to form the quantumwell of the active layer 2 with an extremely small thickness so as toform quantum levels such that the quantum levels are tuned to the 635 nmradiation. This approach, however, is unrealistic because of theadversary surface effects which appears conspicuously in the systemcontaining a very thin quantum well layer, and this is the reason why atensile strain has been used in the active layer of the red-color laserdiode of the related art. In compensation of using a tensile stress forthe active layer, however, one has to suffer from the problem that thelaser beam produced by such a laser diode has a polarization plane ofthe TM mode.

Contrary to the foregoing problematic related art, the active layeraccording to the present invention has a larger bandgap energy and theproblem associated with the excessively small thickness of the quantumwell layer is successfully avoided. Thereby, it is possible to form aquantum well layer with an optimum thickness and the laser diode canproduce the optical radiation at the wavelength of 635 nm by using theactive layer under the desired compressive strain. It should be notedthat the laser beam produced by such a laser diode has the polarizationplane corresponding to the TE mode similarly to other general purposelaser diodes. Thereby, the laser beam can be used in variousapplications without rotating the polarization plane and the cost of theoptical element necessary for rotating the polarization plane isreduced.

In any of the foregoing first through third aspects of the presentinvention, it is preferable to use a GaPAs substrate for the substrate1. In this case, therefore, the heterojunction structure 4 is formed onthe GaPAs substrate. Such a GaPAs substrate can be formed by growing anepitaxial layer of GaPAs on the GaAs substrate 1 with a large thicknessof typically 30 μm by a VPE process. Thereby, the composition of theGaPAs epitaxial layer is controlled such that the GaPAs epitaxial layerhas a lattice constant matching with the lattice constant of theheterojunction structure 4 at the top surface thereof.

Further, in any of the foregoing first through third aspects of thepresent invention, it is preferable to interpose a relaxation bufferlayer 6 between the substrate 1 of GaAs (or GaP) and the cladding layer3 as represented in FIG. 6 so as to absorb the lattice misfit existingbetween the substrate 1 and the cladding layer 3. According to thepresent invention, the heterojunction structure 4 can be formedsuccessfully by an epitaxial process. Thereby, it is advantageous tocarry out the entire epitaxial process starting from the step of formingthe relaxation buffer layer 6 to the step of formation of theheterojunction structure 4 continuously in the same depositionapparatus. The relaxation buffer layer 6 may have a graded compositionsuch that the lattice constant thereof changes gradually from theinterface to the substrate 1 to the interface to the heterojunctionstructure 4. By changing the composition of the relaxation buffer layer6 gradually, the lattice relaxation occurs also gradually and theproblem of the dislocations in the buffer layer 6 penetrating in theupward direction to the heterojunction structure 4 is effectivelyeliminated. Thereby, a high crystal quality is guaranteed for theepitaxial layers constituting the heteroepitaxial structure 4. Therelaxation buffer layer 6 may form a strained superlattice structure inwhich two epitaxial layers having respective lattice constants, such asa GaInP layer and a GaPAs layer, are stacked alternately. By using sucha strained superlattice structure for the relaxation buffer layer 6, itbecomes possible to confine the crystal defects associated with thelattice relaxation in the buffer layer 6. Thereby, the degradation ofcrystal quality of the heterojunction structure 4 is effectivelyeliminated.

Further, it should be noted that the relaxation buffer layer 6 may beformed as a low-temperature buffer layer grown at a temperature lowerthan the temperature used for growing the cladding layer 3. By doing so,it becomes possible to confine the crystal defects associated with thelattice relaxation inside the buffer layer 6, and the problem ofdegradation of the crystal quality of the heteroepitaxial structure 4 iseffectively eliminated.

It should be noted that the foregoing material system including thealternate stacking of the GaPAs layer and the GaInP layer isparticularly suitable for the relaxation buffer layer 6 in view of theternary component system for each of the epitaxial layers. For example,a GaPAs layer can be formed on a GaP substrate by merely adding As toGaP. Similarly, the GaPAs layer can be formed on a GaAs substrate bymerely adding P to GaAs. In the case of forming a GaInP layer, thegrowth of the GaInP layer is easily controlled, particularly at theinterface of the epitaxial layers constituting the strained superlatticestructure, in view of the fact that the system contains P as the onlygroup V element having a large vapor pressure.

In any of the foregoing first through third aspects of the presentinvention, it should be noted that a substrate having an inclinedprincipal surface offset from the (100) surface in the [011] directionby an offset angle in the range of 0-54.7° is preferably used for thesubstrate 1. Alternatively, the principal surface may be inclined in the[0-11] direction by an offset angle in the range of 10-54.7°. Further,the principal surface may be inclined in any equivalent directions. Bydoing so, the spontaneous formation of natural superlattice structure issuppressed, while this contributes to increase the bandgap of theepitaxial layers formed on the substrate. In other words, the use of theso-called offset substrate is useful for tuning the laser oscillationwavelength. Further, the use of the offset substrate suppresses thehillock formation. Thereby, the problem of degradation of deviceperformance or yield of production associated with the hillock formationis successfully eliminated. In the case of fabricating an edge-emissiontype laser diode, the cleaving process for forming an optical cavity hasto be achieved, when such an offset substrate is used, such that thecleaved surfaces defining the optical cavity are also inclined withrespect to the normally used cleaved surface which is perpendicular tothe (100) principal surface.

Further, in any of the first through third aspects of the presentinvention, it is preferable to apply a mechanical polishing process tothe principal surface of the GaPAs substrate after the growth of therelaxation buffer layer 6 but before the growth of the heteroepitaxialstructure 4 so as to eliminate cross-hatch patterns that are frequentlyformed on such a heteroepitaxial substrate as a result of the latticemisfit. It should be noted that such cross-hatch patterns can become thesource of dislocations or other crystal defects in the heteroepitaxialstructure 4, which is essential for the operation of the light-emittingsemiconductor device.

Further, in any of the foregoing first through third aspects of thepresent invention, it is also possible to provide a planarization layeron the semiconductor substrate 1 such that the planarization layer isinterposed between relaxation buffer layer provided on the substrate 1and the heteroepitaxial structure 4, which includes the active layer 2and the cladding layers 3 and may further include the optical waveguidelayers 5. By providing such a planarization layer on the relaxationbuffer layer, the adversary effect of the cross-hatch patterns explainedabove is eliminated. Such a planarization layer may be formed bydepositing a GaInP layer containing Se with a concentration of 5×10¹⁸cm⁻³ or more. More specifically, the inventor of the present inventionhas discovered that such an epitaxial layer of GaInP containing a highconcentration of Se preferentially fills depressions such as the oneformed by the cross-hatch pattern. Thereby, the GaInP layer thuscontaining Se formed a planarized top surface. Further, it should benoted that such GaInP layer containing Se with high concentration can beused also for the relaxation buffer layer 6. By using such a highlySe-doped GaInP layer for the relaxation buffer layer 6, it becomespossible to reduce the thickness of the layer 6.

In any of the foregoing first through third aspects of the presentinvention, it should be noted that the epitaxial layers in theheteroepitaxial structure 4 are formed by either an MOCVD process or anMBE process. On the other hand, formation of these epitaxial layers froma melt is difficult in view of large precipitation coefficient of Alfrom the molten phase to the solid phase. In such a case, the control ofthe composition of the epitaxial layers is difficult. Further, the useof a VPE process based on a halogen transport process is difficult inview of corrosive nature of the source gas AlCl, which tends to reactwith a quartz tube used for the deposition process. Further, the MOCVDprocess or MBE process may also be used for forming the relaxationbuffer layer 6. In this case, the same deposition apparatus can be usedfor forming the relaxation buffer layer 6 and the heteroepitaxialstructure 4 formed thereon.

[First Embodiment]

Hereinafter, a first embodiment of the present invention will bedescribed with reference to FIG. 7 showing the construction of a laserdiode according to the first embodiment in a cross-sectional view.

Referring to FIG. 7, the laser diode has an SCH-SQW structure and isconstructed on an offset GaAs substrate 11 having a principal surfaceinclined from the (100) surface in the [011] direction by an offsetangle of 15°. Thus, a graded buffer layer 12 of n-type GaInP is formedon the substrate 11 by an MOCVD process with a thickness of about 2 μmwhile changing the composition of Ga from 0.5 to 0.7. The graded bufferlayer 12 contains Se with a concentration level of 5×10¹⁸ cm³¹ ³ or moreand provides a lattice relaxation. Further, in order to ensure theplanarization, a further planarization layer 13 of n-type GaInP having auniform composition of Ga_(0.7)In_(0.3)P is formed on the graded bufferlayer 12 with a thickness of about 1 μm. Thereby, any cross-hatchpatterns that may be formed on the surface of the graded buffer layer 12are filled with the planarization layer 13 and a planarized surface isobtained at the top surface of the layer 13.

According to the experiments conducted by the inventor, it wasdiscovered that a GaInP layer doped with Se to a high concentrationlevel (preferably larger than about 5×10¹⁸ cm⁻³) fills the depressionsformed on the graded buffer layer 12 preferentially and that the GaInPlayer 13 thus formed provides an excellent planarized surface, which iseven more flat than the top surface of the GaAs substrate 11. Thereby,the thickness of the planarization layer 13 can be reduced to 1 μm as aresult of high-concentration doping of Se also into the graded bufferlayer 12.

Next, on the planarization layer 13 thus formed, a bottom cladding layer14 of n-type AlInP having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=1, β=0.7, v=1) is formedepitaxially with a thickness of about 1 μm, and a bottom opticalwaveguide layer 15 of undoped AlGaInP having a composition representedas (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0.1, γ=0.7, u=1) isformed epitaxially on the bottom cladding layer 14 with a thickness of0.1 μm. It should be noted that the foregoing bottom cladding layer 14and the bottom optical waveguide layer 15 have a composition thatachieves a lattice matching with the GaInP layer 13 having the foregoingcomposition of Ga_(0.7)In_(0.3)P.

On the optical waveguide layer 15, there is provided a SQW layer 16 ofundoped GaInP having a composition represented as(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (x=0, α=0.6, t=1) as theactive layer of the laser diode, wherein the SQW layer 16 is formedepitaxially with a thickness of about 8 nm. It should be noted that theSQW layer 16 having such a composition accumulates therein a compressivestrain.

Further, a top optical waveguide layer 17 of undoped AlGaInP having acomposition represented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u)(z=0.1, γ=0.7, u=1) is formed on the active layer 16 epitaxially with athickness of about 0.1 μm, and a top cladding layer 18 of p-type AlInPhaving a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=1, β=0.7, v=1) is formedepitaxially on the optical waveguide layer 17 with a thickness of about1 μm. Further, a cap layer 19 of p-type GaInP having a composition ofGa_(0.7)In_(0.3)P is formed epitaxially on the top cladding layer 18with a thickness of about 0.1 μm, and a contact layer 20 of p-type GaAsis formed further thereon epitaxially with a thickness of about 0.005μm. The cladding layers 14 and 18 and the optical waveguide layers 15and 17 have a composition set so as to achieve a lattice matching withthe planarization layer 14 of GaInP, as noted before.

On the contact layer 20, there is provided an insulating layer 21 OfSiO₂ and a p-type ohmic electrode 22 is provided on the insulating layer21 so as to make an ohmic contact with the contact layer 20 at anopening formed in the insulating layer 21. Further, an n-type ohmicelectrode 23 is formed on a bottom surface of the GaAs substrate 11.Thereby, the laser diode forms a stripe laser diode. Of course, theconstruction of FIG. 7 can be modified to form other type of laser diodesuch as a ridge-guide type laser diode.

According to the present embodiment, the cladding layers 14 and 18 orthe optical waveguide layers 15 and 17 have a bandgap larger than thebandgap of the conventional material system that achieves a latticematching with GaAs. Because of this feature of increased bandgapassociated with the strained system, it became possible to realize alaser diode operating at the wavelength of 595 nm, which is shorter thanthe wavelength of 600 nm, while reducing the Al content in the opticalwaveguide layers 15 and 17 as compared with the optical waveguide layersconventionally used for forming a laser diode operable at the wavelengthof 635 nm on a GaAs substrate. Because of the reduced Al content in theoptical waveguide layers 15 and 17, the laser diode of the presentembodiment successfully reduces the non-optical recombination ofcarriers and the efficiency of laser oscillation is improved. Further,associated with the reduction of the Al content, the laser diode of thepresent invention reduces the surface recombination current associatedwith Al, and the laser diode can be operated stably at a high outputpower without inducing a damaging at the edge surface.

In the present embodiment, it should further be noted that the quantumwell layer 16 uses GaInP. As the lattice constant of GaInP increaseswith decreasing Ga content, and in view of the fact that the change ofthe bandgap occurs mainly on the conduction band, not on the valenceband when the Ga content is changed (Sandip, et a., op. cit.), thepresent invention enables to secure a large band discontinuity on theconduction band at the interface between the optical waveguide layer 15or 17 and the quantum well layer 16 while simultaneously reducing the Alcontent in the optical waveguide layer 15 or 17.

In the case of a conventional visible or red laser diode constructed ona GaAs substrate, it has been necessary to use AlGaInP containing alarge amount of Al for the optical waveguide layers 15 and 17 in orderto secure a sufficient band discontinuity at the interface between thequantum well layer 16 and the optical waveguide layer 15 or 17 and havesuffered from various problems associated with Al contained in theoptical waveguide layers.

Thus, the present invention successfully eliminates the problemassociated with the use of Al in the optical waveguide layers 15 and 17while simultaneously securing a large band discontinuity a the interfacebetween the quantum well layer 16 and the optical waveguide layer 15 or17. Thereby, the laser diode of the present embodiment provides anexcellent temperature stability.

In the present embodiment, the cladding layers 14 and 18 are required tohave a bandgap larger than the bandgap of the active layer 16 and mayhave a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1).Further, the optical waveguide layers 15 and 17 are required to have abandgap smaller than the bandgap of the cladding layers 14 and 18 butlarger than the bandgap of the active layer 16. The optical waveguidelayers 15 and 17 are further required to have a lattice constantcoincident to the lattice constant of the cladding layers 14 and 18.Within this limitation, the composition of the optical waveguide layers15 and 17 can be chosen from the composition of(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ<1, 0<u 1).Further, an MQW structure may be used for the active layer 16. Thecompositional parameter β of the cladding layer 14 or 18 influences thelattice constant of the cladding layer, wherein the parameter β is setso that a lattice matching is achieved between the cladding layer 14 or18 and the planarization layer 13 formed at the top part of therelaxation buffer layer 12.

In the laser diode of the present embodiment, the principal surface ofthe GaAs substrate 11 may offset in the [011] direction from the (100)surface with an offset angle within the range of 0 to 54.7° or in the[0-11] direction with an offset angle within the range of 0 to 54.7° forsuppressing the spontaneous formation of natural superlattice structure.As a result of the elimination of the formation of natural superlatticestructure, narrowing of the bandgap is eliminated and the constructionis suitable for decreasing the laser oscillation wavelength. Further,the use of the inclined principal substrate surface is advantageous forreducing the hillock density. Thereby, the problem of deterioration ofthe laser diode performance or yield of production is effectivelyavoided.

In the case of the edge-emission type laser diode as in the example ofFIG. 7, an efficient optical cavity is formed by cleaving the substratewith an inclined angle with respect to the normal cleavage surface,which is perpendicular to the (100) surface, in correspondence to theoffset angle of the inclined principal surface.

In the present embodiment, the entire epitaxial layers including thelayers 12-20 are grown by an MOCVD process in the same depositionchamber by continuing the growth process consecutively and continuously.Thereby, the fabrication cost of the semiconductor device is reducedsignificantly. Alternatively, the epitaxial layers may also be formed byan MBE process.

It should be noted that the foregoing structure of FIG. 7 is alsoapplicable to a light-emitting diode.

[Second Embodiment]

Next, a second embodiment of the laser diode according to the presentinvention will be described again with reference to FIG. 7, wherein thelaser diode of the second embodiment is tuned to the red opticalradiation at the wavelength of 635 nm or 650 nm.

In the present embodiment, the laser diode has a structure substantiallyidentical with the structure of FIG. 7 and the structural descriptionthereof will be omitted.

In the present embodiment, a material in the system of GaInPAscontaining non-infinitesimal amount of As is used for the SQW layerconstituting the active layer 16 for tuning the oscillation wavelengthof the laser diode to red color radiation of the foregoing wavelengths.

More specifically, the GaAs substrate 11 has an inclined principalsurface offset in the [011] direction from the (100) surface by anoffset angle of 15°. Further, the graded buffer layer 12 of Se-dopedGaInP and the planarization layer 13 of Se-doped GaInP are formed on thesubstrate 11 similarly to the previous embodiment, wherein the GaInPplanarization layer 13 has a composition of Ga_(0.7)In_(0.3)P. Similarlyas before, the GaInP layers 12 and 13 are doped with Se to theconcentration level of 5×10¹⁸ cm⁻³.

In the present embodiment, the cladding layers 14 and 18 are formedrespectively of n-type and p-type AlInP both having the compositionrepresented as (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=1, β=0.7,v=1), wherein the layers 14 and 18 are formed to have a thickness ofabout 1 μm, similarly to the previous embodiment. Further, the opticalwaveguide layers 15 and 17 are formed respectively of undoped AlGaInPboth having the composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0.1, γ=0.7, u=1), whereinthe layers 15 and 17 are formed to have a thickness of about 0.1 μm. Onthe other hand, the active layer 16 is formed on an undoped GaInPAshaving a composition represented as(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (x=0, α=0.6, t=0.9), whereinit can be seen that the active layer 16 now contains an effective amountof As. Similarly as before, the active layer 16 may have a thickness ofabout 8 nm. Otherwise, the laser diode of the present invention isidentical with the laser diode of the previous embodiment.

In the foregoing construction of the second embodiment, it should benoted that the cladding layers 14 and 18 of the optical waveguide layers15 and 17 have a composition that achieves a lattice matching with theplanarization layer 13 of GaInP on the top part of the relaxation bufferlayer 12.

According to the present embodiment, the laser diode successfullyoperates at the wavelength of 635 nm as a result of the use of theAs-containing composition for the active layer 16. Further, the activelayer 16 of GaInPAs thus containing As has a composition thataccumulates therein a compressive strain when formed on the GaInP layer13 in the structure of FIG. 7. It should be noted that the a compressivestrain exceeding 1% would be needed in the active layer 16 when theactive layer 16 is formed of GaInP free from Al and when the foregoinglaser oscillation wavelength of 635 nm is to be achieved by thecompressive strain alone. As a result of addition of As, the bandgap ofthe active layer 16 constituting the SQW structure is increased and thelaser oscillation wavelength is tuned to 635 nm. Further, the laserdiode of the present embodiment is advantageous in the practical viewpoint in that the laser diode operates in the TE-mode, like many otherlaser diodes. Thereby, no optical fixture is needed for rotating thepolarization place of the optical beam produced by the laser diodeoperating in the TM-mode.

Further, it is also possible to use a strained quantum well layeraccumulating therein a tensile strain for the active layer 16, as longas the material constituting the quantum well layer has adirect-transition band structure. In this case, it is preferable tochoose the composition of the cladding layers 14 and 18 such that thecladding layers 14 and 18 have a lattice constant larger than thelattice constant of the previous embodiment. Thereby, the bandgap of thecladding layers 14 and 18 or the bandgap of the optical waveguide layers15 and 17 becomes larger as compared with the conventional compositionthat achieves a lattice matching with the GaAs substrate 11, and theband discontinuity at the heteroepitaxial interface is increased and theefficiency of carrier confinement is improved. Further, as a result ofuse of the tensile strain in the cladding layers 14 and 18, it ispossible to use a widegap material for the quantum well layer 16 and thequantum well layer 16 is allowed to have a larger thickness as comparedwith the conventional case of using the cladding layers 14 and 18 thatachieve a lattice matching with the GaAs substrate 11. Further, the Alcontent z in the optical waveguide layers 15 and 17 can be reduced to0.07, and the problem of non-optical recombination of carriers in thelaser diode is substantially reduced. As a result, the efficiency oflaser oscillation is improved and the high-power operation of the laserdiode is improved also.

For the cladding layers 14 and 18, an Al-containing compositionproviding a large bandgap represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1) may beused. As to the optical waveguide layers 15 and 17, a compositionrepresented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1,0.5<γ<1, 0<u≦1) may be used, as long as the optical waveguide has abandgap larger than the bandgap of the active layer 16 but smaller thanthe bandgap of the cladding layers 14 and 18 and as long as the latticeconstant of the optical waveguide layer 15 or 17 is similar to that ofthe cladding layers 14 and 18.

Similarly to the previous embodiment, the laser diode of the presentembodiment also may have an MQW structure for the active layer 16. Thecompositional parameter β of the cladding layers 14 and 18 are adjustedso that the cladding layer achieves a lattice matching with theuppermost layer 13 of the relaxation buffer layer 12. As notedpreviously, the principal surface of the GaAs substrate 11 may be tiltedfrom the (100) surface in the [011] direction within the angle of0-54.7° or in the [0-11] direction within the angle of 10-54.7° forsuppressing the spontaneous formation of the natural superlatticestructure. By doing to, the problem of unwanted decrease of the bandgap,associated with the natural superlattice formation, is avoided. Theepitaxial layers in the structure of FIG. 7 may be formed by any ofMOCVD process of MBE process.

[Third Embodiment]

Next, a third embodiment of the present invention will be describedagain with reference to FIG. 7.

In the present embodiment, the laser diode has a structure similar tothat of FIG. 7 explained already, except that the relaxation bufferlayer 12 is formed of a graded layer of n-type GaPAs in place of theGaInP layer, wherein the content of P is changed gradually in the bufferlayer 2 from 0 at the interface to the substrate 11 to 0.4 at theinterface to the planarization layer 13. The buffer layer 12 is dopedwith Se and is formed with a thickness of about 2 μm. Further, theplanarization layer 13 is now formed of Se-doped n-typeGaP_(0.4)As_(0.6) in place of GaInP, wherein the planarization layer 13is formed on the relaxation buffer layer 12 with a thickness of about 1μm.

[Fourth Embodiment]

Next, a fourth embodiment of the present invention will be describedagain with reference to FIG. 7 and further with reference to FIG. 8,wherein those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIG. 8, it can be seen that the relaxation buffer layer 12is now formed of a strained superlattice structure including analternate stacking of a first GaInP layer having a composition ofGa_(0.7)In_(0.3)P and a second GaInP layer having a composition ofGa_(0.5)In_(0.5)P, wherein the first GaInP layer achieves a latticematching with the cladding layer 14, while the second GaInP layerachieves a lattice matching with the GaAs substrate 11. Each of thefirst and second GaInP layers is doped with Se to a concentration levelof 5×10¹⁸ cm⁻², wherein the first and second GaInP layers are repeatedalternately until the total thickness of the relaxation buffer layer 12becomes about 2 μm.

[Fifth Embodiment]

Next, a fifth embodiment of the present invention will be describedagain with reference to FIG. 7.

In the present embodiment, the relaxation buffer layer 12 of n-typeGaPAs is formed on the foregoing inclined principal surface of the GaAssubstrate 11 with a composition of GaP_(0.4)As_(0.6) by an MOCVD processconducted at a temperature lower than the temperature used for growingthe cladding layer 14. In the present embodiment, the foregoinglow-temperature buffer layer 12 of GaPAs is formed with a thickness ofabout 0.1 μm, and the planarization layer 13 of n-type GaInP doped withSe to the concentration level of 5×10¹⁸ cm⁻³ and having the compositionof Ga_(0.7)In_(0.3)P is formed with a thickness of about 2 μm.

Otherwise, the laser diode of the present embodiment is more or less thesame as the laser diode of previous embodiment and further descriptionthereof will be omitted.

[Sixth Embodiment]

Next, a sixth embodiment of the laser diode according to the presentinvention will be described again with reference to FIG. 7 and furtherwith reference to FIG. 9, wherein the laser diode of the secondembodiment is tuned to the red optical radiation at the wavelength of635 nm or 650 nm. In FIG. 9, those parts corresponding to the partsdescribed previously are designated by the same reference numerals andthe description thereof will be omitted.

In the present embodiment, the laser diode has a structure substantiallyidentical with the structure of FIG. 7 and the structural descriptionthereof will be omitted.

In the present embodiment, an MQW structure is used for the active layer16.

More specifically, the GaAs substrate 11 has an inclined principalsurface offset in the [011] direction from the (100) surface by anoffset angle of 15°. Further, the graded buffer layer 12 of Se-dopedGaInP and the planarization layer 13 of Se-doped GaInP are formed on thesubstrate 11 similarly to the previous embodiment, wherein the GaInPplanarization layer 13 has a composition of Ga_(0.7)In_(0.3)P. Similarlyas before, the GaInP layers 12 and 13 are doped with Se to theconcentration level of 5×10¹⁸ cm⁻³.

In the present embodiment, the cladding layers 14 and 18 are formedrespectively of n-type and p-type AlGaInP both having the compositionrepresented as (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.7, β=0.7,v=1), wherein the layers 14 and 18 are formed to have a thickness ofabout 1 μm. Further, the optical waveguide layers 15 and 17 are formedrespectively of undoped GaInP both having the Al-free compositionrepresented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.7,u=1), wherein the layers 15 and 17 are formed to have a thickness ofabout 0.1 μm. On the other hand, the active layer 16 has an MQWstructure represented in FIG. 9 wherein a number of undoped GaPAsquantum well layers 16 a each having a thickness of 8 nm and acomposition represented as (Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t)(x=0, α=1, t=0.3) and a number of undoped Al-free barrier layers 16 b ofGaInP each having a thickness of 10 nm and a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.7, u=1) are stackedalternately and repeatedly. Otherwise, the laser diode of the presentinvention is identical with the laser diode of the previous embodiment.The quantum well layers 16 a in such a construction accumulates thereina compressive strain.

In the present embodiment, which uses the foregoing compositions for thecladding layers 14 and 18, it is necessary to induce a substantialstrain in the quantum well layers 16 a in order to achieve a laseroscillation at the wavelength of 650 nm, even in such a case GaInP,which is the material that provides the smallest bandgap in the AlGaInPfamily, is used for the quantum well layer 16 a. In order to avoid thedegradation of quality of the crystal constituting the quantum welllayer 16 a, the quantum well layer 16 a of the present embodiment isadded with As so as to tune the bandgap thereof to the desiredwavelength without using excessive strain. As noted already, the bandgapof GaInP decreases with decreasing Ga content. In view of Sandip opcit., such a change of bandgap occurs mainly on the conduction band. Nosubstantial change occurs on the valence band. Thus, in theheteroepitaxial system formed of an optical waveguide layer of Al-freeGaInP and a compressed quantum well layer of GaInP, the banddiscontinuity appears mostly on the conduction band. Thereby, theproblem of carrier (electron) overflow, which has been a problem in ared laser diode formed of the AlGaInP material system, on the conductionband is effectively eliminated. While such a structure may raise aconcern about the hole confinement on the valence band, theincorporation of As into the GaInP system causes a change of bandstructure mainly on the valence band, and the system that uses theGaInPAs quantum well layer 16 a in combination with the opticalwaveguide layer or cladding layer of GaInP realizes an effective carrierconfinement into the quantum well layer 16 a, in which the desiredstimulated emission takes place.

The laser diode of the present embodiment oscillates at the wavelengthof 650 nm and produces a polarized beam having the TE-mode. Theefficiency of carrier confinement is improved over the case in which amaterial system that achieves a lattice matching to the GaAs substrateis used, due to the increased bandgap of the cladding layers 14 and 18or the optical waveguide layers 15 and 17. Further, the MQW structureused in the present embodiment contributes to increase the carrierconfinement into the quantum well layers 16 a further. As a result ofthe use of compressive strain in the quantum well layers 16 a, thepresent embodiment allows the use of a widegap material for the quantumwell layers when tuning the oscillation wavelength of the laser diode tothe desired red optical wavelength. Thereby, the thickness of thequantum well layers 16 a can be increased as compared with the case inwhich a material system that achieves a matching to the GaAs substrate 1is used for the cladding layer 14 or 18, and the adversary effect of thesurface is reduced. As the quantum well layers 16 a, the barrier layers16 b and the optical waveguide layers 15 and 17 are free from Al, theproblem of non-optical recombination of carriers and associated problemof decrease of the efficiency of laser oscillation is also eliminated.Further, the problem of damaging at the free edge surface of the lasercavity caused by Al contained in the optical waveguide layers 15 and 17or in the active layer 16 is eliminated and the laser diode can beoperated stably at a high output power.

In the present embodiment, the cladding layers 14 and 18 may also havean Al-containing composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1),provided that the cladding layers have a bandgap energy larger than thebandgap energy of the active layer 16. The cladding layers 14 and 18change the lattice constant when the compositional parameter β ischanged, wherein the compositional parameter β is adjusted so that thecladding layer 14 has a lattice constant matching with the latticeconstant of planarization layer 13 or the top part of the relaxationbuffer layer 12. Further, the barrier layers 16 b may have a compositionso as to accumulate strain therein. For example, the barrier layers 16 bmay be formed so as to accumulate strain opposite to the strainaccumulated in the quantum well layers 16 a. This construction is usefulfor compensating for the strain of the quantum well layers 16 aparticularly in the case the strain accumulated in the quantum welllayers 16 is excessively large.

Otherwise, the present embodiment is similar to those describedpreviously.

[Seventh Embodiment]

Next, a seventh embodiment of the present invention will be describedagain with reference to FIGS. 7 and 9, wherein the present embodimentprovides a laser diode operable at the wavelength of-red opticalradiation such as 635 nm or 650 nm while using Al-free optical waveguidelayers.

In the present embodiment, an offset GaP substrate is used in place ofthe GaAs substrate 1, wherein the offset GaP substrate 1 has an inclinedprincipal surface inclined by an offset angle of 15° from the (100)direction in the [011] direction. On the inclined principal surface ofthe GaP substrate 1, there is provided a graded buffer layer of n-typeGaInP in place of the buffer layer 12 by an MOCVD process while changingthe Ga content from 1 to 0.78. The graded buffer layer 12 of GaInP isdoped with Se to a concentration level of 5×10¹⁸ cm⁻³ or more and isformed with a thickness of about 2 μm. Further, a planarization layer ofn-type GaInP is formed in correspondence to the planarization layer 13with a thickness of about 1 μm, wherein the planarization layer 13 has acomposition of Ga_(0.78)In_(0.22)P and is doped with Se to aconcentration level of 5×10¹⁸ cm⁻³ or more. By doping the planarizationlayer 13 with Se to a high concentration level, the planarization layer13 fills any cross-hatch depressions formed on the top surface of thebuffer layer 12 and provides a planarized top surface. It should benoted that the planarized top surface of the layer 13 has a superiorflatness as compared with the top surface of the GaP substrate 11. Thus,the use of the planarization layer is 13 is quite effective foreliminating the defect formation in the epitaxial layers grown on thesubstrate 11.

On the planarization layer 13, there are formed upper and lower claddinglayers respectively of n-type and p-type AlGaInP corresponding to thecladding layers 14 and 18 explained previously, wherein both of thecladding layers 14 and 18 thus formed have the composition representedas (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.7, β=0.78, v=1) andhave a thickness of about 1 im. Further, optical waveguide layerscorresponding to the optical waveguide layers 15 and 17 are formed so asto having an Al-free composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.78, u=1), whereinthe optical waveguide layers 15 and 17 thus formed have a thickness ofabout 0.1 μm. Further, an active layer corresponding to the active layer16 is formed between the upper and lower optical waveguide layers 15 and17, wherein the active layer 16 of the present embodiment has an MQWstructure similar to the one represented in FIG. 9 wherein three undopedGaPAs quantum well layers 16 a each having a thickness of 8 nm and acomposition represented as (Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t)(x=0, α=1, t=0.3) and three undoped Al-free barrier layers 16 b of GaInPeach having a thickness of 10 nm and a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.78, u=1) are stackedalternately. Otherwise, the laser diode of the present invention isidentical with the laser diode of the previous embodiment. The quantumwell layers 16 a in such a construction accumulates therein acompressive strain.

In the present embodiment, which uses the foregoing compositions for thecladding layers 14 and 18, it is necessary to induce a substantialstrain in the quantum well layers 16 a in order to achieve a laseroscillation at the wavelength of 635 nm, even in such a case GaInP,which is the material that provides the smallest bandgap in the AlGaInPfamily, is used for the quantum well layer 16 a. In order to avoid thedegradation of quality of the crystal constituting the quantum welllayer 16 a, the quantum well layer 16 a of the present embodiment isadded with As so as to tune the bandgap thereof to the desiredwavelength without using excessive strain.

The laser diode of the present embodiment oscillates at the wavelengthof 650 nm and produces a polarized beam having the TE-mode. Theefficiency of carrier confinement is improved over the case in which amaterial system that achieves a lattice matching to the GaAs substrateis used, due to the increased bandgap of the cladding layers 14 and 18or the optical waveguide layers 15 and 17. Further, the MQW structureused in the present embodiment contributes to increase the carrierconfinement into the quantum well layers 16 a further. As a result ofthe use of compressive strain in the quantum well layers 16 a, thepresent embodiment allows the use of a widegap material for the quantumwell layers when tuning the oscillation wavelength of the laser diode tothe desired red optical wavelength. Thereby, the thickness of thequantum well layers 16 a can be increased as compared with the case inwhich a material system that achieves a matching to the GaAs substrate 1is used for the cladding layer 14 or 18, and the adversary effect of theinterface is reduced. As the quantum well layers 16 a, barrier layers 16b or the optical waveguide layers 15 and 17 are free from Al, theproblem of non-optical recombination of carriers and associated problemof decrease of the efficiency of laser oscillation is also eliminated.Further, the problem of damaging at the free edge surface of the lasercavity caused by Al contained in the optical waveguide layers 15 and 17or in the active layer 16 is eliminated and the laser diode can beoperated stably at a high output power.

In the present embodiment, the cladding layers 14 and 18 may also havean Al-containing composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β<1, 0<v≦1),provided that the cladding layers have a bandgap energy larger than thebandgap energy of the active layer 16. The cladding layers 14 and 18change the lattice constant when the compositional parameter β ischanged, wherein the compositional parameter β is adjusted so that thecladding layer 14 has a lattice constant matching with the latticeconstant of planarization layer 13 or the top part of the relaxationbuffer layer 12. Further, the barrier layers 16 b may have a compositionso as to accumulate strain therein. For example, the barrier layers 16 bmay be formed so as to accumulate strain opposite to the strainaccumulated in the quantum well layers 16 a. This construction is usefulfor compensating for the strain of the quantum well layers 16 aparticularly in the case the strain accumulated in the quantum welllayers 16 is excessively large.

Otherwise, the present embodiment is similar to those describedpreviously.

[Eighth Embodiment]

FIG. 10 shows the construction of a laser diode according to an eighthembodiment of the present invention.

Referring to FIG. 10, the laser diode has an SCH-SQW structure and isconstructed on a so-called GaPAs substrate 34.

More specifically, an offset GaAs substrate 31 having a principalsurface inclined from the (100) surface in the [011] direction by anoffset angle of 15° is used in the present embodiment and a gradedbuffer layer 32 of n-type GaPAs is formed on the GaAs substrate 31 by aVPE process with a thickness of about 30 μm while changing thecomposition of P gradually from 0 to 0.4. Further, a planarization layer33 of n-type GaPAs having a uniform composition of GaP_(0.4)As_(0.6) isformed on the graded buffer layer 32 with a thickness of about 20 μm.Thereby, the graded buffer layer 32 and the planarization layer 33 form,together with the GaAs substrate 31, the GaPAs substrate 34.

Next, on the GaPAs substrate 34 thus formed, a buffer layer 35 of n-typeGaInP doped with Se and having a composition of Ga_(0.7)In_(0.3)P isformed with a thickness of about 1 μm, and a bottom cladding layer 36 ofn-type AlInP having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=1, β=0.7, v=1) is formedepitaxially on the foregoing buffer layer 35 with a thickness of about 1μm. Further, a bottom optical waveguide layer 37 of undoped AlGaInPhaving a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0.1, γ=0.7, u=1) is formedepitaxially on the bottom cladding layer 36 with a thickness of 0.1 μm.It should be noted that the foregoing bottom cladding layer 36 and thebottom optical waveguide layer 37 have a composition that achieves alattice matching with the GaPAs substrate 34 having the foregoingcomposition of GaP_(0.4)As_(0.6).

On the optical waveguide layer 37, there is provided an SQW layer 38 ofundoped GaInP having a composition represented as(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (x=0, α=0.55, t=1) as theactive layer of the laser diode, wherein the SQW layer 38 is formedepitaxially with a thickness of about 8 nm. It should be noted that theSQW layer 38 having such a composition accumulates therein a compressivestrain when formed on the foregoing GaPAs substrate 34.

Further, a top optical waveguide layer 39 of undoped AlGaInP having acomposition represented as (Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u)(z=0.1, γ=0.7, u=1) is formed on the active layer 16 epitaxially with athickness of about 0.1 μm, and a top cladding layer 40 of p-type AlInPhaving a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=1, β=0.7, v=1) is formedepitaxially on the optical waveguide layer 39 with a thickness of about1 μm. Further, a cap layer 41 of p-type GaInP having a composition ofGa_(0.7)In_(0.3)P is formed epitaxially on the top cladding layer 40with a thickness of about 0.1 μm, and a contact layer 42 of p-type GaAsis formed further thereon epitaxially with a thickness of about 0.005μm. The cladding layers 36 and 40 and the optical-waveguide layers 37and 39 have a composition set so as to achieve a lattice matching withthe planarization layer 33 at the top part of the GaPAs substrate 34, asnoted before.

On the contact layer 42, there is provided an insulating layer 43 OfSiO₂ and a p-type ohmic electrode 44 is provided on the insulating layer43 so as to make an ohmic contact with the contact layer 42 at anopening formed in the insulating layer 43. Further, an n-type ohmicelectrode 45 is formed on a bottom surface of the GaAs substrate 31.Thereby, the laser diode forms a stripe laser diode. Of course, theconstruction of FIG. 10 can be modified to form other type of laserdiode such as a ridge-guide type laser diode.

According to the present embodiment, a laser diode operable at thewavelength of 635 nm is obtained, wherein it should be noted that thelaser diode of the present embodiment produces a laser beam with aTE-polarization mode. Due to the increased bandgap for the claddinglayers 36 and 40 or for the optical waveguide layers 37 and 39, theefficiency of carrier confinement into the active layer 38 is improvedover the laser diode that uses a composition matching to the GaAssubstrate for the cladding layers or for the waveguide layers. Further,associated with the increased bandgap for the SQW layer 38 as a resultof accumulation of the compressive strain therein, the thickness of theSQW layer 38 can be increased as compared with the SQW layer thatachieves a lattice matching with the GaAs substrate while simultaneouslymaintaining the desired tuning of the laser oscillation wavelength tothe foregoing wavelength of 635 nm. Thereby, the adversary effect ofsurface states on the stimulated emission taking place in the SQW layer38 is minimized. Further, associated with the decrease of the Al contentin the optical waveguide layers 37 and 39 as compared with theconventional laser diodes for use in the same wavelength band, theproblem of damaging occurring at the free edge surface of the laseroptical cavity associated with the non-optical recombination centersformed by Al, is successfully eliminated and the laser diode can beoperated at a high optical output power with excellent reliability.

In the present embodiment, the cladding layers 36 and 40 are required tohave a bandgap larger than the bandgap of the active layer 38 and mayhave a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β≦1, 0<v≦1).Further, the optical waveguide layers 37 and 39 are required to have abandgap smaller than the bandgap of the cladding layers 36 and 40 butlarger than the bandgap of the active layer 38. The optical waveguidelayers 37 and 39 are further required to have a lattice constantcoincident to the lattice constant of the cladding layers 36 and 40.Within this limitation, the composition of the optical waveguide layers37 and 39 can be chosen from the composition of(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (0≦z<1, 0.5<γ<1, 0<u≦1).Further, an MQW structure may be used for the active layer 38. Thecompositional parameter β of the cladding layer 36 or 40 influences thelattice constant thereof, wherein the parameter β is set so that alattice matching is achieved between the cladding layers 36 and 40 andthe planarization layer 33 at the top part of the GaPAs substrate 34.

In the laser diode of the present embodiment, the principal surface ofthe GaAs substrate 11 may offset in the [011] direction from the (100)surface with an offset angle within the range of 0 to 54.7° or in the[0-11] direction within the range of 10 to 54.7° for suppressing thespontaneous formation of natural superlattice structure. As a result ofthe elimination of the formation of natural superlattice structure,narrowing of the bandgap is eliminated and the construction is suitablefor decreasing the laser oscillation wavelength.

[Ninth Embodiment]

FIG. 11 shows the construction of a laser diode according to a ninthembodiment of the present invention.

Referring to FIG. 11, the laser diode has an SCH-SQW structure and isconstructed on a so-called GaPAs substrate 54.

More specifically, an offset GaP substrate 51 having a principal surfaceinclined from the (100) surface in the [011] direction by an offsetangle of 15° is used in the present embodiment and a graded buffer layer52 of n-type GaPAs is formed on the GaAs substrate 51 by a VPE processwith a thickness of about 30 μm while changing the composition of Pgradually from 1 to 0.55. Further, a planarization layer 53 of n-typeGaPAs having a uniform composition of GaP_(0.55)As_(0.45) is formed onthe graded buffer layer 52 with a thickness of about 20 μm. Thereby, thegraded buffer layer 52 and the planarization layer 53 form, togetherwith the GaAs substrate 51, the GaPAs substrate 54.

In the present embodiment, the top surface of the GaPAs substrate 54thus formed is subjected to a mechanical polishing process so as toeliminate the cross-hatch patterns that tend to develop in such astrained heteroepitaxial system.

After etching the surface of the GaPAs substrate 54 slightly by a wetetching process for removing damaged portions, a buffer layer 55 ofn-type GaInP having a composition of Ga_(0.78)In_(0.22)P is formed onthe GaPAs substrate 54. Further, a bottom cladding layer 56 of n-typeAlInP having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.7, β=0.78, v=1) is formedepitaxially on the foregoing buffer layer 55 with a thickness of about 1μm. Further, a bottom optical waveguide layer 57 of undoped GaInP havingan Al-free composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, ã=0.78, u=1) is formedepitaxially on the bottom cladding layer 56 with a thickness of 0.1 μm.It should be noted that the foregoing bottom cladding layer 56 and thebottom optical waveguide layer 57 have a composition that achieves alattice matching with the GaPAs substrate 54 having the foregoingcomposition of GaP_(0.4)As_(0.6).

On the optical waveguide layer 57, there is provided an active layer 58having an MQW structure, wherein three undoped GaPAs quantum well layerseach having a thickness of 8 nm and a composition represented as(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (x=0, α=1, t=0.3) and threeundoped Al-free barrier layers of GaInP each having a thickness of 10 nmand a composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.78, u=1) are stackedalternately. In the active layer 58, it should be noted that the quantumwell layers of the foregoing composition accumulate therein acompressive strain.

Further, a top optical waveguide layer 59 of undoped GaInP having aAl-free composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.78, u=1) is formedon the active layer 58 epitaxially with a thickness of about 0.1 μm, anda top cladding layer 60 of p-type AlInP having a composition representedas (Al_(y)Ga_(1-y))_(a)In_(1-β)P_(v)As_(1-v) (y=0.7, β=0.78, v=1) isformed epitaxially on the optical waveguide layer 59 with a thickness ofabout 1 μm. Further, a cap layer 61 of p-type GaInP having a compositionof Ga_(0.7)In_(0.3)P is formed epitaxially on the top cladding layer 60with a thickness of about 0.1 μm, and a contact layer 62 of p-type GaAsis formed further thereon epitaxially with a thickness of about 0.005μm. The cladding layers 56 and 60 and the optical waveguide layers 57and 59 have a composition set so as to achieve a lattice matching withthe top part of the GaPAs substrate 54 of the compositionGa_(0.78)In_(0.22)P, as noted before.

On the contact layer 62, there is provided an insulating layer 63 OfSiO₂ and a p-type ohmic electrode 64 is provided on the insulating layer63 so as to make an ohmic contact with the contact layer 62 at anopening formed in the insulating layer 63. Further, an n-type ohmicelectrode 65 is formed on a bottom surface of the GaAs substrate 51.Thereby, the laser diode forms a stripe laser diode. Of course, theconstruction of FIG. 11 can be modified to form other type of laserdiode such as a ridge-guide type laser diode.

In the laser diode of the present embodiment, the quantum well layers inthe active layer 58 are added with As so as to adjust the bandgap forobtaining the laser oscillation at the wavelength of 635 nm in view ofthe fact that an excessive strain would be needed if an As-freecomposition is used for the quantum well layers for producing the laseroscillation at the foregoing wavelength of 635 nm.

According to the present embodiment, a laser diode operable at thewavelength of 635 nm is obtained, wherein it should be noted that thelaser diode of the present embodiment produces a laser beam with aTE-polarization mode. Due to the increased bandgap for the claddinglayers 56 and 60 or for the optical waveguide layers 57 and 59, theefficiency of carrier confinement into the active layer 58 is improvedover the laser diode that uses a GaAs lattice-matching composition forthe cladding layers or for the waveguide layers. Further, associatedwith the increased bandgap for the quantum well layers in the activelayer 58 as a result of accumulation of the compressive strain therein,the thickness of the quantum well layers can be increased as comparedwith the case in which a composition that achieves a lattice matchingwith the GaAs substrate is used while simultaneously maintaining thedesired tuning of the laser oscillation wavelength to the foregoingwavelength of 635 nm. Thereby, the adversary effect of surface states onthe stimulated emission taking place in the quantum well layers in theactive layer 38 is minimized. Further, associated with the Al-freecomposition for the active layer 58 and for the optical waveguide layers57 and 59, the problem of damaging occurring at the free edge surface ofthe laser optical cavity associated with the non-optical recombinationcenters formed by Al, is successfully eliminated and the laser diode canbe operated at a high optical output power with excellent reliability.

In the present embodiment, the cladding layers 46 and 60 are required tohave a bandgap larger than the bandgap of the active layer 58 and mayhave a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<yò1, 0.5<β<1, 0<v≦1). Thecompositional parameter β of the cladding layer 56 or 60 influences thelattice constant thereof, wherein the parameter β is set so that alattice matching is achieved between the cladding layer 56 and 60 andthe top part of the GaPAs substrate 54.

In the laser diode of the present embodiment, the principal surface ofthe GaAs substrate 51 may offset in the [011] direction from the (100)surface with an offset angle within the range of 0 to 54.7° forsuppressing the spontaneous formation of natural superlattice structure.As a result of the elimination of the formation of natural superlatticestructure, narrowing of the bandgap is eliminated and the constructionis suitable for decreasing the laser oscillation wavelength.

[Tenth Embodiment]

FIG. 12 shows the construction of a laser diode according to an eighthembodiment of the present invention.

Referring to FIG. 12, the laser diode has an SCH-SQW structure and isconstructed on a so-called GaPAs substrate 74.

More specifically, an offset GaAs substrate 71 having a principalsurface inclined from the (100) surface in the [110] direction by anoffset angle of 2° is used in the present embodiment and a graded bufferlayer 72 of n-type GaPAs is formed on the GaAs substrate 71 by a VPEprocess with a thickness of about 30 μm while changing the compositionof P gradually from 0 to 0.4. Further, a planarization layer 73 ofn-type GaPAs having a uniform composition of GaP_(0.4)As_(0.6) is formedon the graded buffer layer 32 with a thickness of about 20 μm. Thereby,the graded buffer layer 72 and the planarization layer 73 form, togetherwith the GaAs substrate 71, the GaPAs substrate 74. At the top part ofthe GaPAs substrate, the lattice misfit with respect to the GaAssubstrate 71 is effectively relaxed and the substrate 74 can be regardedas a single substrate having a composition of GaP_(0.4)As_(0.6).

Next, on the GaPAs substrate 74 thus formed, a buffer layer 75 of n-typeGaInP doped with Se and having a composition of GaP_(0.4)As_(0.6) isformed with a thickness of about 1 μm, and a bottom cladding layer 76 ofn-type AlInPAs having an As-containing composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8, v=0.85) isformed epitaxially on the foregoing buffer layer 75 with a thickness ofabout 1 μm. Further, a bottom optical waveguide layer 77 of undopedGaInP having an Al-free composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.7, u=1) is formedepitaxially on the bottom cladding layer 76 with a thickness of 0.1 μm.It should be noted that the foregoing bottom cladding layer 76 and thebottom optical waveguide layer 77 have a composition that achieves alattice matching with the GaPAs substrate 74 having the foregoingcomposition of GaP_(0.4)As_(0.6).

On the optical waveguide layer 77, there is provided an SQW layer 78 ofundoped GaInPAs having an Al-free composition represented as(Al_(x)Ga_(1-x))_(α)In_(1-α)P_(t)As_(1-t) (x=0, α=0.65, t=0.9) as theactive layer of the laser diode, wherein the SQW layer 78 is formedepitaxially with a thickness of about 25 nm. It should be noted that theSQW layer 78 having such a composition accumulates therein a compressivestrain when formed on the foregoing GaPAs substrate 74.

Further, a top optical waveguide layer 79 of undoped AlGaInP having anAl-free composition represented as(Al_(z)Ga_(1-z))_(γ)In_(1-γ)P_(u)As_(1-u) (z=0, γ=0.7, u=1) is formed onthe active layer 78 epitaxially with a thickness of about 0.1 μm, and atop cladding layer 80 of p-type AlGaInPAs having a compositionrepresented as (Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (y=0.5, β=0.8,v=0.85) is formed epitaxially on the optical waveguide layer 79 with athickness of about 1 μm. Further, a cap layer 81 of p-type GaInP havinga composition of Ga_(0.7)In_(0.3)P is formed epitaxially on the topcladding layer 80 with a thickness of about 0.1 μm, and a contact layer82 of p-type GaAs is formed further thereon epitaxially with a thicknessof about 0.005 μm. The cladding layers 76 and 80 and the opticalwaveguide layers 77 and 79 have a composition set so as to achieve alattice matching with the GaPAs substrate 74, as noted before.

On the contact layer 82, there is provided an insulating layer 83 ofSiO₂ and a p-type ohmic electrode 84 is provided on the insulating layer83 so as to make an ohmic contact with the contact layer 82 at anopening formed in the insulating layer 83. Further, an n-type ohmicelectrode 85 is formed on a bottom surface of the GaAs substrate 71.Thereby, the laser diode forms a stripe laser diode. Of course, theconstruction of FIG. 12 can be modified to form other type of laserdiode such as a ridge-guide type laser diode.

According to the present embodiment, a laser diode operable at thewavelength of 666 nm is obtained.

In the laser diode of the present embodiment, the principal surface ofthe GaAs substrate 71 is offset slightly in the [011] direction from the(100) surface with a small offset angle of 2°. When a layer of theAlGaInP system is grown on such a slightly offset GaAs substrate by wayof an MOCVD process, there occurs an extensive hillock formation on thesubstrate surface, wherein the degree of hillock formation is enhancedwhen the Al content is large in the epitaxial layer grown on thesubstrate. Such a hillock formation provides a disastrous effect on thedevice performance or yield of production of the laser diode.

Meanwhile, the present inventor has discovered that the hillockformation can be successfully suppressed by incorporating As into theAlGaInP layer thus grown on the substrate. It is believed that As thusintroduced effectively suppress the droplet formation of Ga or Al on thesurface of the substrate. Further, the laser diode of the presentembodiment, not containing Al in the optical waveguide layers 77 and 79or in the quantum well layer 78, is advantageous for improving theefficiency of laser oscillation. Further, the laser diode of the presentembodiment can stably produce a high output optical power.

In the present embodiment, too, it is possible to replace the SQW layer78 with an MQW layer such as the one described with reference to 11.

Further, the laser diodes described heretofore can be used also forforming a light-emitting diode.

[Eleventh Embodiment]

Next, a laser diode according to an eleventh embodiment of the presentinvention will be described.

Generally, a laser diode is formed with a current confinement structurefor effectively confining the carriers injected into the laser diode sothat the carriers are concentrated along an axial region of the activelayer so that efficient stimulated emission takes place along such anaxial region. In order to achieve such a current confinement, it iscommonly practiced to form a ridge structure in the semiconductorlayered body of the laser diode such that the ridge structure extends inthe axial direction of the laser diode. Alternatively, a stripestructure is formed so as to extend the layered body of the laser diodein the axial direction thereof.

In order to form such a ridge current confinement structure or a stripecurrent confinement structure, it is necessary to apply an etchingprocess to the layered body of the laser diode, while such an etchingprocess uses an etching stopper for stopping the etching process at thedesired depth or desired location. In order to facilitate the productionof the laser diode, it is preferable to form the etching stopper layersin the same process for forming the epitaxial layers of the laser diodeby using the same family of materials while adjusting the compositionsuch that the etching stopper layer shows a resistance against theetching used for etching other epitaxial layers.

In view of the fact that the etching stopper layer used for such apurpose in a laser diode does not contribute to the laser oscillation,it is desired that the etching stopper layer does not absorb the opticalradiation produced in the laser diode. For this purpose, it is desiredthat the etching stopper layer has a large bandgap. In the laser diodethat uses the III-V system of AlGaInPAs, such as the one producing redoptical radiation, there is a wide possibility of selection of materialsfor increasing the bandgap of the etching stopper layer by merelyincreasing the Al content. However, increase of Al content in such anetching stopper layer generally increases the etching rate thereof andthe etching stopper layer thus containing a large amount of Al may notfunction as an efficient etching stopper.

Thus, it has been practiced to induce a tensile strain in such anetching stopper so that the absorption optical wavelength thereof isoffset from the wavelength of the laser oscillation.

However, such an approach has a drawback in that the thickness of theetching stopper layer has to be held within the critical thicknessthereof, while the use of such a thin etching stopper layer tends tocause the problem of non-uniform etching. This problem becomesparticularly serious in visible wavelength laser diodes that produces anoptical beam with a wavelength such as 633 nm or shorter. In such acase, a strained quantum well structure is needed for the etchingstopper layer, wherein the desired thickness of the quantum well layercan become several nanometers or less. Otherwise, it is necessary toincrease the strain to 1% or more, while such the critical thicknessbecomes very small in such a highly strained epitaxial layer.

Thus, the present embodiment provides a laser diode structure thataddresses the foregoing problems of the related art.

FIG. 13 shows the structure of a laser diode according to an eleventhembodiment of the present invention.

Referring to FIG. 13, the laser diode is constructed on a substrate 101of n-type GaAs and includes a graded buffer layer 102 of n-type GaPAs inwhich the As content therein is changed from 1 at the bottom surfacecontacting the GaAs substrate 101 to 0.6 at the top surface.

On the graded buffer layer 102, there is formed a buffer layer 103 ofn-type GaPAs having a composition of GaAs_(0.6)P_(0.4), and a bottomcladding layer 104 of n-type AlGaInP is formed epitaxially on the bufferlayer 103. The bottom cladding layer 104 has a composition that achievesa lattice matching with the top part of the graded buffer layer 102.Further, an active layer 105 of undoped GaPAs or GaInPAs is formedepitaxially on the bottom cladding layer 104 and a top cladding layer106 of p-type AlGaInP is formed epitaxially further on the active layer105.

In the present embodiment, it should be noted that the top claddinglayer 106 is covered with an etching stopper layer 107 of p-type GaInPepitaxially, and a further cladding layer 108 of p-type AlGaInP isformed epitaxially on the etching stopper layer 107.

Further, an anti-spike layer 109 of p-type GaInP and a contact layer 110of p-type GaAs are formed consecutively on the cladding layer 108.

The contact layer 110, the anti-spike layer 109 and the cladding layer108 are then patterned by a wet etching process while using an SiO₂pattern (not shown) as an etching mask, to form a current-confiningridge structure extending axially through the semiconductor layered bodyof the laser diode. In this wet etching process, the contact layer 110is first etched by a sulfuric etchant until the anti-spike layer 109 isexposed. Upon the exposure of the anti-spike layer 109, the etchant isswitched to hydrochloric acid and the anti-spike layer 109 is etched.Further, the etchant is switched to the sulfuric etchant again and thecladding layer 108 of p-type AlGaInP is etched, until the etchingstopper layer 107 is exposed. Upon exposure of the etching stopper layer107, the wet etching process stops spontaneously.

After the formation of the forgoing ridge structure, an insulating layer111 of SiO₂ is deposited and a p-type electrode 112 is deposited on theinsulating layer 111 such that the p-type electrode 112 makes an ohmiccontact with the contact layer 110 at the top part of the ridgestructure via an opening formed in the insulating layer 111.

In the present embodiment, it should be noted that the etching stopperlayer 107 of p-type GaInP has a composition that achieves a latticematching to the buffer layer 103 of GaAs_(0.6)P_(0.4), and the etchingstopper layer 107 can have a large thickness of as large as 50 nmwithout inducing creation of dislocations therein. Thereby, the etchingstopper layer 107 functions as a reliable etching stopper and theproblem of non-uniform patterning of the ridge structure, which tends toarise when an extremely thin etching stopper is used, is effectivelyeliminated.

In the present embodiment, it should be noted that the etching stopperlayer 107 has a bandgap wavelength of 560 nm, which is shorter than thebandgap wavelength of the GaPAs active layer 105. Thus, the problem ofabsorption of the optical radiation by the etching stopper layer 107does not occur in the laser diode of the present embodiment.

In the present embodiment, it is possible to form a visible laser diodethat produces red optical radiation in the wavelength range of 630-660nm by using a GaInP mixed crystal for the active layer 105 as notedbefore. The etching stopper layer 107 of GaInP does not absorb theoptical radiation of the foregoing wavelength band.

[Twelfth Embodiment]

FIG. 14 shows the construction of a laser diode according to a twelfthembodiment of the present invention, wherein those parts correspondingto the parts described previously are designated by the same referencenumerals and the description thereof will be omitted. It should be notedthat the present embodiment addresses the same problem explained in theprevious embodiment with reference to FIG. 13.

Referring to FIG. 14, the laser diode has an SCH-SQW structure andincludes optical waveguide layers 201 and 203 interposed between theactive layer 105 and the bottom cladding layer 104 or between the activelayer 105 and the top cladding layer 106. In the present embodiment, theactive layer 105 has an Al-free composition of GaInPAs tuned to the redoptical wavelength, while the optical waveguide layers 201 and 203 havean Al-free composition in the system of GaInP.

Further, the laser diode of FIG. 14 uses a pair of current confinementregions 204 of n-type GaPAs at both lateral sides of the ridgestructure, wherein the current confinement regions 204 have acomposition of GaAs_(0.6)P_(0.4). In other words, the currentconfinement regions 204 have a lattice constant that matches the latticeconstant of the buffer layer 103.

As the current-confinement regions 204 of n-type GaPAs forms the desiredcurrent confinement together with the ridge structure of the p-type, thelaser diode of FIG. 14 does not require an insulating layer such as thelayer 111 used in the previous embodiment, and the p-type contact layer110 of GaAs covers the regions 204 and the ridge structure uniformly.Further, the p-type electrode 112 covers the contact layer 110uniformly.

As explained with reference to the previous embodiments, the laser diodeof the present embodiment, which lacks Al in the optical waveguidelayers 201 and 203 can operate stably at high output power withoutcausing damage at the cavity edge surface.

Again, the problem of optical absorption by the etching stopper layer107 is successfully eliminated in the present embodiment while securinga sufficient thickness for the etching stopper layer.

As is well known in the art, the current-confinement regions 204 at bothlateral sides of the central ridge structure induces a waveguide loss byabsorbing the optical radiation leaking outside the ridge structure, andthe optical radiation produced in the active layer 105 is effectivelyguided along the central ridge structure.

As the current-confinement structure 204 is formed of GaPAs, which is anAl-free composition, as noted previously, no substantial oxidationoccurs at the surface thereof, and the contact layer 110 can be grownthereon with high quality.

[Thirteenth Embodiment]

Next, a laser diode according to a thirteenth embodiment of the presentinvention will be described with reference to FIG. 15, wherein thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.It should be noted that the present embodiment addresses the sameproblem explained in the previous embodiment with reference to FIG. 13or 14.

Referring to FIG. 15, the laser diode has a layered structure similar tothat of FIG. 13, except that the current confinement regions 204 isformed in the form of a layer having a central opening extending in theaxial direction of the laser diode, and the cladding layer 108 isprovided on the current confinement layer 204 so as to make a contactwith the exposed GaInP etching stopper layer 107 at the foregoingopening. The Cladding layer 108 is further covered with the anti-spikelayer 108 and the contact layer 109 consecutively, and the p-type ohmicelectrode 110 covers the top surface of the contact layer 110. Further,the n-type ohmic electrode 113 is formed on the bottom surface of theGaAs substrate 101.

In this construction, too, the GaInP layer 107 functions as an effectiveetching stopper when forming the central opening in the currentconfinement layer 204 by a wet etching process. Thereby, a mixture ofsulfuric acid, hydrogen peroxide and water may be used for the etchant.Due to the large bandgap of GaInP, a sufficient thickness can be securedfor the layer 107 without causing optical absorption of the optical beamproduced in the active layer 105. It is not necessary to introducestrain in the etching stopper layer 107.

As is well known in the art, the current-confinement regions 204 at bothlateral sides of the central opening induces a waveguide loss byabsorbing the optical radiation leaking laterally from the central axialregion of the laser diode. Thereby, the device operates as a stablesingle-mode laser diode even when operated at a large optical outputpower.

In view of the fact that the current-confinement structure 204 having anAl-free composition, the epitaxial layers 108-110 can be grown thereonwith high-quality.

[Fourteenth Embodiment]

FIG. 16 shows the construction of a laser diode according to afourteenth embodiment of the present invention, wherein those partscorresponding to the part described previously will be designated by thesame reference numerals and the description thereof will be omitted. Itshould be noted that the present embodiment addresses the same problemexplained in the previous embodiments with reference to FIGS. 13-15.

In the present embodiment, a current-confinement structure 601 of n-typeAlInP is used in place of the current-confinement structure 204 ofGaPAs, and the optical absorption by the current-confinement structure204 is eliminated. In such a construction, the external differentialquantum efficiency of the laser diode is improved.

In order to facilitate the growth of further epitaxial layers on thecurrent-confinement structure 601, the current-confinement structure,which now contains Al, is covered with a cap layer 602 of n-type GaPAs.

It should be noted that the structure of FIG. 16 is also effective forguiding the optical beam along the central axial region of the laserdiode in view of the fact that the regions 601 of AlInP have a lowerrefractive index as compared with the cladding layer 106 or 108 ofAlGaInP.

In any of the foregoing embodiments of FIGS. 13-16, it should be notedthat GaInP etching stopper layer 107 functions as an effective etchingstopper layer also when a mixture of hydrochloric acid and phosphoricacid or a mixture of sulfuric acid, hydrogen peroxide and water is usedfor the etchant.

[Fifteenth Embodiment]

Next, a laser diode according to a fifteenth embodiment of the presentinvention will be described with reference to FIGS. 17-19.

In a laser diode or a light-emitting semiconductor device that producesan optical radiation with a wavelength of 530 nm to visible wavelengthrange, particularly in the wavelength range of red optical radiation of630-680 nm, the confinement of carriers, particularly the electrons inthe active layer is an important factor for improving the efficiency oflight emission, as explained already with reference to FIGS. 4A and 4B.

In order to improve the efficiency of carrier confinement, JapaneseLaid-Open Patent Publication 4-114486 proposes a barrier layer in thep-type cladding layer so as to block the electrons overflowing from theactive layer. The reference further proposes to use a multiple quantumbarrier (MQB) structure for the barrier layer. A MQB structure enhancesthe apparent barrier height by inducing a multiple reflection ofelectron waves in a superlattice structure. Such an MQB structure isalso proposed in the Japanese Laid-Open Patent Publication 7-235733 withregard to the AlGaInP system.

On the other hand, such conventional proposal has been made withreference to a laser diode using a material system that achieves alattice matching to GaAs. In other words, the laser diodes of theserelated art are not the device for producing visible wavelength opticalradiation.

Thus, the technology of providing such a carrier blocking barrier layerin the laser diode operable in the visible wavelength band has not beenestablished.

In the preceding embodiments, description has been made with regard tothe use of a GaPAs substrate having an effective lattice constantbetween the lattice constant of GaAs and the lattice constant of GaP,for the substrate of visible optical wavelength laser diode operating inthe red color wavelength band.

FIG. 17 shows the band diagram showing a part of the band structure ofthe AlGaInP system of FIG. 1.

Referring to FIG. 17, it can be seen that various widegap materials areavailable in the system of AlGaInP when the composition thereof ischosen offset from the lattice matching composition to GaAs.

Thus, the present embodiment provides a laser diode constructed on aGaPAs substrate and operable at the wavelength of 630-660 nm, whereinsuch an MQB carrier blocking layer is provided in the cladding layer forincreasing the efficiency of carrier confinement.

FIG. 18 shows the construction of the laser diode according to thepresent embodiment.

Referring to FIG. 18, the laser diode is constructed on an n-type GaAssubstrate 701 covered with a buffer layer 702 of a GaPAs material,wherein the buffer layer 702 has a composition changing gradually fromGaAs to GaP_(0.3)As_(0.7).

On the GaPAs buffer layer 702, there is provided a bottom cladding layer703 of n-type AlGaInP having a composition represented as (Al_(a1),Ga_(1-a1))_(b1)In_(1-b1)P (0.51<b1<1), wherein the compositionalparameter a1 may be set to 1 and the compositional parameter b1 may beset to 0.66 in a typical example. The parameters a1 and b1 aredetermined so that the cladding layer 703 achieves a lattice matchingwith the GaPAs buffer layer 702. In other words, the cladding layer 703has a lattice constant intermediate between the lattice constant of GaAsand the lattice constant of GaP.

Further, a bottom optical waveguide layer 704A of undoped AlGaInP havinga composition represented as (Al_(a2),Ga_(1-a2))_(b2)In_(1-b2)P (a2<a1)is formed epitaxially on the bottom cladding layer 703, wherein thecompositional parameter a2 may be set to 0.15 and the compositionalparameter b2 may be set equal to b1 (b2=b1).

Further, an active layer 705 of undoped GaInPAs is formed epitaxially onthe bottom optical waveguide layer 704A and a top optical waveguidelayer 704B of undoped AlGaInP is formed epitaxially on the opticalwaveguide layer 705 with the composition identical with the compositionof the optical waveguide layer 704A.

In the present embodiment, an MQB layer 706 is further provided on thetop optical waveguide layer 704B as an electron blocking structure,wherein the MQB layer 706 includes an alternate repetition of a barrierlayer of p-type AlGaInP having a composition of(Al_(a1),Ga_(1-a1))_(b1)In_(1-b1)P (0.51<b1<1) identical with thecomposition of the cladding layer 703 and a quantum well layer of p-typeAlGaInP having a composition of (Al_(a6),Ga_(1-a6))_(b6)In_(1-b6)P(a₆=0.15, b6=b1), and a top cladding layer 707 of p-type AlGaInP isprovided further on the MQB layer 706 with the composition identicalwith the composition of the bottom cladding layer 703.

Further, an intermediate layer 708 of p-type GaInP and a contact layer709 of p-type GaPAs are formed consecutively on the top cladding layer707 with respective compositions that achieve a lattice matching withthe GaPAs buffer layer 702.

The layers 707-709 are subjected to a patterning process to form acentral ridge structure extending in the axial direction of the laserdiode, and is covered with an insulating layer 710 of SiO₂. Further, ap-type electrode 711 is provided on the insulating layer 710 so as tomake an ohmic contact with the contact layer 709 at a contact openingformed in the insulating layer 710 in correspondence to the centralridge structure.

In the illustrated example, the cladding layers 703 and 707 and theoptical waveguide layers 704A and 704B have a lattice constant offset byabout −1% with respect to the lattice constant of GaAs.

FIG. 19 shows the band structure of the laser diode of FIG. 18.

Referring to FIG. 19, it can be seen that there is formed a quantum wellin correspondence to the active layer 705 and there occurs stimulatedemission in the active layer 705 as a result of recombination ofelectrons and holes confined in the active layer 705, wherein theelectrons are injected from the side of the bottom cladding layer 703,while the holes are injected from the side of the top cladding layer707. Further, FIG. 19 indicates that there is formed a multiple quantumwell structure in correspondence to the MQB layer 706, wherein the MQBlayer 706 induces a multiple reflection of electrons therein andeliminates the escaping of the electrons from the active layer 705 tothe top cladding layer 707. It should be noted that the MQB layer 706forms an effective barrier structure against electrons. The quantum welllayers 706 b of the MQB structure 706 may accumulate strain as long asthe thickness thereof does not exceed the critical thickness.

In one example the laser diode of the present embodiment showed a lowthreshold for laser oscillation at the wavelength of 630-636 nm.Further, the laser diode showed a characteristically small temperaturedependence, indicating the improved carrier confinement into the activelayer 705.

[Sixteenth Embodiment]

FIG. 20 shows the construction of a laser diode according to a sixteenthembodiment of the present invention, wherein those parts correspondingto the parts described previously are designated by the same referencenumerals and the description thereof will be omitted. It should be notedthat the present embodiment addresses the same problem explained in theprevious embodiment with reference to FIGS. 17-19.

Referring to FIG. 20, the laser diode is now constructed on a GaPAssubstrate 719 of n-type having a composition representedGaAs_(b4)P_(1-b4) (b4=0.7), wherein the GaPAs substrate 719 is coveredwith a GaPAs buffer layer 720 of n-type having the same composition asthe GaPAs substrate 719.

On the buffer layer 720, the bottom cladding layer 703 of n-type AlGaInPis formed with the composition of (Al_(a1),Ga_(1-a1))_(b1)In_(1-b1)P,with the compositional parameters a1 and b1 set to 0.25 and 0.66,respectively.

In the present embodiment, a bottom carrier blocking layer 714 of n-typeAlGaInP is provided on the bottom cladding layer 703 with a compositionrepresented as (Al_(a5),Ga_(1-a5))_(b5)In_(1-b5)P, wherein thecompositional parameters a5 is set to 0.8 and the compositionalparameter b5 is set equal to the compositional parameter b1 (b5=b1).

Further, the bottom optical waveguide layer 704A of undoped AlGaInP isformed on the carrier blocking layer 714 with the compositionrepresented as (Al_(a2),Ga_(1-a2))_(b2)In_(1-b2)P, with thecompositional parameter a2 being set equal to 0.15 and the compositionalparameter b2 set equal to the compositional parameter b1 (b2=b1).

On the bottom optical waveguide layer 704A, the active layer 705 ofundoped GaInPAs is formed, and the top optical waveguide layer 704B ofundoped AlGaInP having the composition identical with the composition ofthe optical waveguide layer 704A is formed on the active layer 705.

Further, a top carrier blocking layer 713 of p-type AlGaInP is formed onthe optical waveguide layer 704B with the composition identical with thecomposition of the bottom carrier blocking layer 714, and the topcladding layer 707 of p-type AlGaInP having the same composition as thebottom cladding layer 703 is formed on the top carrier blocking layer713. On the top carrier blocking layer 713, the intermediate layer 708and the contact layer 709 are formed similarly to the device of theprevious embodiment.

FIG. 21 shows the band structure of the laser diode of FIG. 21.

Referring to FIG. 21, it can be seen that there are formed a pair ofpotential barriers outside the optical waveguide layers 704A and 704B,wherein the potential barriers formed by the layers 713 and 714effectively blocks the escaping of the electrons and holes injected intothe active layer 705. The carrier blocking layers 713 and 714 are formedtypically with a thickness of 50 nm or more for eliminating thetunneling of the carriers therethrough. As the carrier blocking layer714 is doped to the p-type, the barrier height at the conduction band Evis enhanced and the efficiency of electron confinement is furtherimproved.

It should be noted that the laser diode of the present embodiment has arefractive index distribution symmetric with respect to the active layer705. The laser diode oscillates efficiently at the wavelength of 630-636nm.

[Seventeenth Embodiment]

Next, a laser diode according to a seventeenth embodiment of the presentinvention will be described with reference to FIG. 22, wherein thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.It should be noted that the present embodiment also addresses theproblem explained in the previous embodiment with reference to FIGS.17-19.

Referring to FIG. 22, the laser diode has a construction similar to thatof the laser diode of FIG. 20 except that an n-type GaP substrate 715 isused in place of the GaAs substrate 701 or GaPAs substrate, and acorresponding buffer layer 721 of n-type GaPAs is used in place of thebuffer layer 702 or 720. The buffer layer 721 changes a compositionthereof gradually from GaP at the side of the substrate 715 toGaP_(0.6)As_(0.4). Further, the laser diode of the present embodimentuses a carrier blocking layer 716 of p-type AlGaInP at the interfacebetween the top optical waveguide layer 704B and the top cladding layer707, wherein the carrier blocking layer 716 has a compositionrepresented as (Al_(a5),Ga_(1-a5))_(b5)In_(1-b5)P, with thecompositional parameters a5 and b5 set to 0.8 and 0.92 respectively andis formed with a thickness of about 25 nm.

In the present embodiment, the cladding layers 703 and 707 and theoptical waveguide layers 704A and 704B have a lattice misfit of about−2% with respect to the lattice constant of GaAs, wherein the latticemisfit with respect to the GaP substrate 715 is effectively relaxed bythe buffer layer 721.

On the other hand, the carrier blocking layer 716 of the foregoingcomposition has a lattice misfit of about −1% with respect to thecladding layer 707 and accumulates therein a tensile strain. As a resultof the tensile strain, the potential barrier formed in the conductionband Ec by the carrier blocking layer 716 is enhanced with respect tothe active layer 705 as represented in the band diagram of FIG. 23, andthe efficiency of electron confinement is further improved.

Referring to the band diagram of FIG. 23, it can be seen that thepotential barrier associated with the carrier blocking layer 716 isformed inside the top cladding layer 707.

[Eighteenth Embodiment]

FIG. 24 shows the band diagram of the laser diode according to anineteenth embodiment of the present invention wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to the band diagram of FIG. 24, the laser diode has aconstruction similar to that of FIG. 22 except that the carrier blockinglayer 716 is replaced with an MQB layer 717.

As can be seen from FIG. 24, the MQB layer 717 includes three barrierlayers 717 a of p-type AlGaInP having a composition so as to accumulatea tensile strain of about 1% or more with respect to the cladding layer707. On the other hand, quantum well layers 717 b intervening betweenthe barrier layers 717 a have a composition identical with the claddinglayer 707.

The construction of FIG. 24 is also effective for inducing an apparentpotential barrier of electrons at the side of the p-type cladding layer707.

[Nineteenth Embodiment]

FIG. 25 shows the band diagram of a laser diode according to anineteenth embodiment of the present invention, wherein those partsdescribed previously with reference to preceding drawings are designatedby the same reference numerals and the description thereof will beomitted. It should be noted that the present embodiment also addressesthe problem explained in the previous embodiment with reference to FIGS.17-19.

Referring to FIG. 25, the laser diode has an MQB layer 718 includingthree barrier layers 718 a and intervening quantum well layers 718 bboth of p-type AlGaInP, wherein the barrier laye4rs 718 a have a largerbandgap energy than the cladding layer 707 and a composition generallyachieve a lattice matching with respect to the cladding layer 707. Onthe other hand, the quantum well layers 718 b have a compositionidentical with the composition of the optical waveguide layer 704B.

The laser diode of the present embodiment also oscillates at the redoptical wavelength of 630-633 with an improved efficiency as comparedwith the case where no such carrier blocking layer is provided.

Further, the present invention is by no means limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

What is claimed is:
 1. An electronic apparatus having a light-emittingsemiconductor device, said light-emitting semiconductor devicecomprising: a semiconductor substrate; an active layer provided abovesaid semiconductor substrate, said active layer producing a red opticalradiation; and a cladding layer provided above said semiconductorsubstrate adjacent to said active layer, said active layer comprising aIII-V material in a system of AlGaInPAs having a composition representedas (Al_(x)Ga_(1-x)) _(α)In_(1-α)P_(t)As_(1-t)(0≦x<1, 0<α≦1, 0≦t≦1), saidcladding layer containing Al and comprising a III-V material in a systemof AlGaInPAs having a composition represented as(Al_(y)Ga_(1-y))_(β)In_(1-β)P_(v)As_(1-v) (0<y≦1, 0.5<β≦1, 0≦v≦1), andsaid cladding layer having a bandgap larger than a bandgap of saidactive layer and a lattice constant intermediate between a latticeconstant of GaP and a lattice constant of GaAs, said cladding layerhaving said lattice constant and a thickness set outside a range inwhich said cladding layer can grow on any of a GaP crystal and a GaAscrystal without forming a misfit dislocation.
 2. An electronic apparatusas claimed in claim 1, wherein said electronic apparatus is a laserprinter.
 3. An electronic apparatus as claimed in claim 1, wherein saidelectronic apparatus is an optical wiring apparatus.
 4. An electronicapparatus as claimed in claim 1, wherein said electronic apparatus is acompact disc apparatus.
 5. An electronic apparatus as claimed in claim1, wherein said electronic apparatus is a DVD apparatus.